wal: add a bench for write entry

MacAir with ssd
```
BenchmarkWrite100EntryWithoutBatch	   10000	    116255 ns/op
BenchmarkWrite100EntryBatch10	  200000	     12764 ns/op
BenchmarkWrite100EntryBatch100	  500000	      3165 ns/op
BenchmarkWrite100EntryBatch500	 1000000	      1777 ns/op
BenchmarkWrite100EntryBatch1000	 1000000	      1561 ns/op

BenchmarkWrite1000EntryWithoutBatch	   10000	    124229 ns/op
BenchmarkWrite1000EntryBatch10	  100000	     20692 ns/op
BenchmarkWrite1000EntryBatch100	  200000	      9441 ns/op
BenchmarkWrite1000EntryBatch500	  500000	      7273 ns/op
BenchmarkWrite1000EntryBatch1000	  500000	      7048 ns/op
```

@yichengq so probably we can batch around 1000 entries(at most 20ms), which is around diminishing return point, for a fsync. 
Write100ByteEntryWithoutBatch