[common] use temp intead of {filePath}.new

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

common/os.go

@@ -93,9 +93,8 @@ func MustWriteFile(filePath string, contents []byte, mode os.FileMode) {
 	}
 }
 
-// Writes to newBytes to filePath.
-// Guaranteed not to lose *both* oldBytes and newBytes,
-// (assuming that the OS is perfect)
+// WriteFileAtomic writes newBytes to temp and atomically moves to filePath
+// when everything else succeeds.
 func WriteFileAtomic(filePath string, newBytes []byte, mode os.FileMode) error {
 	// If a file already exists there, copy to filePath+".bak" (overwrite anything)
 	if _, err := os.Stat(filePath); !os.IsNotExist(err) {
@@ -108,13 +107,27 @@ func WriteFileAtomic(filePath string, newBytes []byte, mode os.FileMode) error {
 			return fmt.Errorf("Could not write file %v. %v", filePath+".bak", err)
 		}
 	}
-	// Write newBytes to filePath.new
-	err := ioutil.WriteFile(filePath+".new", newBytes, mode)
+	f, err := ioutil.TempFile("", "")
 	if err != nil {
-		return fmt.Errorf("Could not write file %v. %v", filePath+".new", err)
+		return err
+	}
+	_, err = f.Write(newBytes)
+	if err == nil {
+		err = f.Sync()
+	}
+	if closeErr := f.Close(); err == nil {
+		err = closeErr
+	}
+	if permErr := os.Chmod(f.Name(), mode); err == nil {
+		err = permErr
+	}
+	if err == nil {
+		err = os.Rename(f.Name(), filePath)
+	}
+	// any err should result in full cleanup
+	if err != nil {
+		os.Remove(f.Name())
 	}
-	// Move filePath.new to filePath
-	err = os.Rename(filePath+".new", filePath)
 	return err
 }