(Rocks) DB WAL adds overheads while using the DB for saving checkpoints
for block storage. Avoiding writing to WAL translates the write workload
(appending blocks to the blockfile) into a sequential disk writes.

This commit changes the way checkpoints are saved - checkpoints are
written to DB as before, however since WAL is disabled, the checkpoint
stays in-memory only. The in-memory checkpoint is flushed explicitly to
disk (via DB flush) at the time of new block file creation. The in-memory
checkpoint would implicitly also be flushed to memory at the time of DB
shutdown. However, if a crash takes place, the checkpoint available in the
DB would be behind the actual block file status. In order to handle this
crash scenario, this commit also adds code to perform a scan of the block
file beyond the last saved checkpoint and update the checkpoint.

Change-Id: Ie88646b225abaa50b595833f5e7ed8d4facae920
Signed-off-by: manish <manish.sethi@gmail.com>