I assume there is a deeper issue with how UnmarshalBinary works in
go-amino (i.e., when loading array of some objects, the empty array
becomes []object{nil}). Note when Marshaling, the object is nil.
save transactions to blockstore
move to a separate module
benchmark KVIndexer
batch write transactions
Benchmarks:
```
BenchmarkKVIndexerIndex-2 100000 516300 ns/op
PASS
ok github.com/tendermint/tendermint/blockchain/tx 56.506s
5,16 s for 10000 transactions
1 s for 2000 transactions
```
```
BenchmarkKVIndexerIndex-2 h 3000000 8622 ns/op
PASS
ok github.com/tendermint/tendermint/blockchain/tx 34.210s
86 ms for 10000 transactions
16 ms for 2000 transactions
```
```
BenchmarkKVIndexerIndex1-2 5000000 7160 ns/op
BenchmarkKVIndexerIndex500-2 20000 1750411 ns/op
BenchmarkKVIndexerIndex1000-2 10000 3573973 ns/op
BenchmarkKVIndexerIndex2000-2 5000 7836851 ns/op
BenchmarkKVIndexerIndex10000-2 1000 33438980 ns/op
PASS
ok github.com/tendermint/tendermint/blockchain/tx 209.482s
7,8 ms for 2000 transactions
```
[state] write test for ApplyBlock
review comments
- move txindexer to state
- fix type
save Tx Index as well
do not store tx itself in the result