wal: add a bench for write entry

MacAir with ssd
```
BenchmarkWriteEntryWithoutBatch	   10000	    114508 ns/op
BenchmarkWriteEntryBatch10	  200000	     11347 ns/op
BenchmarkWriteEntryBatch100	 1000000	      2613 ns/op
BenchmarkWriteEntryBatch500	 2000000	      1282 ns/op
BenchmarkWriteEntryBatch1000	 2000000	       935 ns/op
```

@yichengq so probably we can batch around 1000 entries(at most 20ms), which is around diminishing return point, for a fsync. 
b.ResetTimer()