fdb_rollback() needs to be supported at file-level

It may be necessary to support rollback at a file level to ensure that in a multi-kv instance mode, we rollback all the kv instances to a specified point in which each of the instance is consistent with the others based on their commit.

Example,
There are 2 kv instance A and B with one key each reflecting their account balance, and 2 transactions happen moving $5 from account A to account B
-----commit: A=10,B=10--------commit: A=5,B=15----------commit: A=0,B=20
Now if user only issues rollback on kv instance B to the first point, then we have
------A=0,B=10
This may be an inconsistent state with respect to the application's use-case.

Therefore, for consistency, forestdb should support fdb_rollback() as a file-level operation where all kv instances are rollbacked atomically.