Feature: Add a write ahead log to handle DB failures

[TheIllusionistMirage]

To handle external failures like OS crashes etc, an append only write ahead log needs to be added. For each incoming write, the write will first be performed on this write ahead log, and then on the currently active memtable. This will ensure that in case of crashes we can consistently rebuild the active memtable. This write ahead log is discarded and a new one created whenever the active memtable is flushed to the disk.