We can make the implementation more robust by adjusting our assumptions
and leverage explicit file modes for syncing. Additionally we going to
assume that we want to clean up and can't really recover if thos
operations (file close and removal) fail.

* utilise file mode for majority of concerns
* improve test coverage by covering more assumptions
* signature parity with ioutil.WriteFile
* always clean up

Replaces #160

otherwise it could try to execute go binary and panic if no go binary
found. See https://github.com/tendermint/tendermint/issues/782

Currently IsDirEmpty returns true, err if it encounters
any error after trying to os.Open the directory.
I noticed this while studying the code and recalled a bug
from an earlier project in which doing the exact same thing
on code without permissions would trip out and falsely report
that the directory was empty.
Given demo.go in https://play.golang.org/p/vhTPU2RiCJ

* Demo:
```shell
$ mkdir -p sample-demo/1 && touch sample-demo/2
$ echo "1st round" && go run demo.go sample-demo
$ sudo chown root sample-demo && sudo chmod 0700 sample-demo
$ echo "2nd round" && go run demo.go sample-demo
```

That then prints out
```shell
1st round
original:: empty: false err: <nil>
updated::  empty: false err: <nil>
2nd round
original:: empty: true err: open data/: permission denied
updated::  empty: false err: open data/: permission denied
```

where in "2nd round", the original code falsely reports that
the directory is empty but that's a permission error.

I could write a code test for it, but that test requires me to change
users and switch to root as a Go user so no point in complicating our
tests, but otherwise it is a 1-to-1 translation between shell and Go.

We use WriteFileAtomic in two places:

```
p2p/addrbook.go
338:    err = cmn.WriteFileAtomic(filePath, jsonBytes, 0644)

types/priv_validator.go
162:    err = WriteFileAtomic(privVal.filePath, jsonBytes, 0600)
```

and we don't need .bak in any of the above. We save priv_validator every
10ms and addrbook every 2 min.

The problem with {filePath}.new is that it is not safe for concurrent
use! Calling this function with the same params results in the following
error:

```
panic: Panicked on a Crisis: rename /root/.tendermint_test/consensus_replay_test/priv_validator.json.new /root/.tendermint_test/consensus_replay_test/priv_validator.json: no such file or directory

goroutine 47860 [running]:
github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common.PanicCrisis(0xcba800, 0xc42152d640)
	/go/src/github.com/tendermint/tendermint/vendor/github.com/tendermint/tmlibs/common/errors.go:33 +0x10f
github.com/tendermint/tendermint/types.(*PrivValidator).save(0xc42235f2c0)
	/go/src/github.com/tendermint/tendermint/types/priv_validator.go:165 +0x159
github.com/tendermint/tendermint/types.(*PrivValidator).signBytesHRS(0xc42235f2c0, 0x6, 0x0, 0xc424e88f03, 0xc429908580, 0xca, 0x155, 0x80, 0xc424e88f00, 0x7f4ecafc88d0, ...)
	/go/src/github.com/tendermint/tendermint/types/priv_validator.go:249 +0x2bb
github.com/tendermint/tendermint/types.(*PrivValidator).SignVote(0xc42235f2c0, 0xc4228c7460, 0xf, 0xc424e88f00, 0x0, 0x0)
	/go/src/github.com/tendermint/tendermint/types/priv_validator.go:186 +0x1a2
github.com/tendermint/tendermint/consensus.(*ConsensusState).signVote(0xc424efd520, 0xc400000002, 0xc422d5e3c0, 0x14, 0x20, 0x1, 0xc4247b6560, 0x14, 0x20, 0x0, ...)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1556 +0x35e
github.com/tendermint/tendermint/consensus.(*ConsensusState).signAddVote(0xc424efd520, 0x2, 0xc422d5e3c0, 0x14, 0x20, 0x1, 0xc4247b6560, 0x14, 0x20, 0xc42001b300)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1568 +0x200
github.com/tendermint/tendermint/consensus.(*ConsensusState).enterPrecommit(0xc424efd520, 0x6, 0x0)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1082 +0x13a4
github.com/tendermint/tendermint/consensus.(*ConsensusState).addVote(0xc424efd520, 0xc424e88780, 0x0, 0x0, 0x39, 0x1dc, 0x28c)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1477 +0x1be5
github.com/tendermint/tendermint/consensus.(*ConsensusState).tryAddVote(0xc424efd520, 0xc424e88780, 0x0, 0x0, 0xd7fb00, 0xc42152ce00)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:1382 +0x93
github.com/tendermint/tendermint/consensus.(*ConsensusState).handleMsg(0xc424efd520, 0xcb58e0, 0xc42547feb8, 0x0, 0x0, 0x6, 0x0, 0x4, 0xed10ca07e, 0x3077bfea, ...)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:660 +0x9fc
github.com/tendermint/tendermint/consensus.(*ConsensusState).receiveRoutine(0xc424efd520, 0x0)
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:615 +0x5f5
created by github.com/tendermint/tendermint/consensus.(*ConsensusState).OnStart
	github.com/tendermint/tendermint/consensus/_test/_obj_test/state.go:332 +0x4a7
exit status 2
FAIL	github.com/tendermint/tendermint/consensus	76.644s
make: *** [test_integrations] Error 1
```

See https://github.com/tendermint/tendermint/pull/568