We need to recover the state DB when
(i) the peer fails during commit (partial written state).
(ii) the database gets corrupted.

We introduce a new state in DB called savepoint, and update
it with the block height once all valid tx in the last/recent block is
committed.

Whenever peer starts (first boot up or after a failure), we compare the
savepoint in DB and block height to see whether the state DB is in
consistent state. If not, we execute the following steps:
(i) retrieve all required blocks (#blocks = block height - savepoint)
from block storage,
(ii) compute write set for valid tx, commits these values, & update the
savepoint.

Change-Id: I769d1391de511d3cdb55c40692beb829e2cc5c2f
Signed-off-by: senthil <cendhu@gmail.com>