BUG-1074: add javadoc for QueueKeeper

- added javadoc for QueueKeeper and QueueKeeperImpl

(cherry picked from commit 24e0f734efdafca6b8cc1ffce7767d0d6bce9fae)
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Signed-off-by: Michal Rehak <mirehak@cisco.com>
Change-Id: I5d81de545fd66e9ea2ed44882245dbee59c48a5b