[common] do not create {filePath}.bak in WriteFileAtomic
[common] do not create {filePath}.bak in WriteFileAtomic

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.

tendermint/tendermint,tendermint/tendermint,tendermint/tendermint,tendermint/tendermint