add wal record buffer

[crwen]

#189

Now data wiil be appended to file every time encoding. This PR add a simple buffer to cache record encoded data when write WAL.

Here are the results of a test on my machine(macbook pro 18) with 10000 DynRecords inserted

without buffer	with buffer	wal disabled
7.747s	0.714s	0.160s

[ethe]

Could it be able to become a general buffered writer in fusio? Always using 4KB or 8KB as a chunk, flush to disk, Record bounding type makes it not general.