Refactored wal code

[merlimat]

Existing WAL implementation is based on external library that was imported here for convenience. The main problems with that implementation are:

• Keeping a cache of the segments entirely in memory
• When a segment is accessed, the whole file is scanned and inserted in memory
• The memory usage when there are many shards in a server can get quite high (eg: ~2 GB for ~16 shards)
• There is significant GC overhead (the buffers are kept for quite some time in memory and then thrown away)
• Trimming/Truncating the log implies making expensive copies of the segments, instead of just aligning the trimming at the segment level.

Implemented simple wal mechanism using mmap to have efficient cached access for multiple readers, without using excessive memory.