Also lazily load the commits to only be run once when
the benchmarks are activated, lest it slows down all the tests

Updates #1021

* Implement a GetHeightBinarySearch method that looks for
the height using the binary search algorithm guaranteeing
worst case iteration time of O(log2(n))
whereas
worst case iteration time of O(n) for the current linear search

So if n we had 500 commits stored by height and sorted, to
trigger the worst case scenario for each, pass in
the most negative height you can find e.g. -1
Linear search: 500 iterations
Binary search: 9 iterations

with n=1000, qHeight = -1
Linear search: 1000 iterations
Binary search: 10 iterations

with n=1e6, qHeight = -1
Linear search: 1e6 iterations
Binary search: 20 iterations

Of course there are realistic expectations e.g. a max of
commits that may be saved so linear search might be useful
for very small size set because it has less preparing overhead
and only ~2 types of comparisons, but nonetheless binary search
shines as soon as we start to hit say 50 commits to search from
as you can see below:

```shell
$ go test -v -run=^$ -bench=MemStore
goos: darwin
goarch: amd64
pkg: github.com/tendermint/tendermint/lite
BenchmarkMemStoreProviderGetByHeightLinearSearch5-4     300000        6491 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch50-4      200000       12064 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch100-4      50000       32987 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch500-4       5000      395521 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch1000-4	       500     2940724 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch5-4     300000        6281 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch50-4      200000       10117 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch100-4     100000       18447 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch500-4      20000       89029 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch1000-4	      5000      265719 ns/op      1600 B/op       15 allocs/op
PASS
ok    github.com/tendermint/tendermint/lite 86.614s
$ go test -v -run=^$ -bench=MemStore
goos: darwin
goarch: amd64
pkg: github.com/tendermint/tendermint/lite
BenchmarkMemStoreProviderGetByHeightLinearSearch5-4     300000        6779 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch50-4      100000       12980 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch100-4      30000       43598 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch500-4       5000      377462 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightLinearSearch1000-4	       500     3278122 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch5-4     300000        7084 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch50-4      200000        9852 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch100-4     100000       19020 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch500-4      20000       99463 ns/op      1600 B/op       15 allocs/op
BenchmarkMemStoreProviderGetByHeightBinarySearch1000-4	      5000      259293 ns/op      1600 B/op       15 allocs/op
PASS
ok    github.com/tendermint/tendermint/lite 86.204s
```

which gives
```shell
$ benchstat old.txt new.txt
name                               old time/op    new time/op    delta
MemStoreProviderGetByHeight5-4       6.63µs ± 2%    6.68µs ± 6%   ~             (p=1.000 n=2+2)
MemStoreProviderGetByHeight50-4      12.5µs ± 4%    10.0µs ± 1%   ~             (p=0.333 n=2+2)
MemStoreProviderGetByHeight100-4     38.3µs ±14%    18.7µs ± 2%   ~             (p=0.333 n=2+2)
MemStoreProviderGetByHeight500-4      386µs ± 2%      94µs ± 6%   ~             (p=0.333 n=2+2)
MemStoreProviderGetByHeight1000-4    3.11ms ± 5%    0.26ms ± 1%   ~             (p=0.333 n=2+2)
```

If need be we can make a hybrid algorithm that switches between the
linear and binary search depending on the number of items.
This is reminiscent of Python's TimSort algorithm.

Update state to keep track of this info.
Change function args as needed.
Make NumTx also an int64 for consistency.

uint64 is considered dangerous. the details will follow in a blog post.

The certifier package is renamed to light. This is more descriptive
especially in the wider blockchain context. Moreover we are building
light-clients using the light package.

This also adds godocs to all exported functions.

Furthermore it introduces some extra error handling. I've added one TODO
where I would like someone else's opinion on how to handle the error.