mds: process finished contexts in batch

mds: process finished contexts in batch

If there are several unstable locks in an inode, current Locker::eval(CInode*,)
processes each lock's finished contexts seperately. This may cause very deep
call stack if finished contexts also call Locker::eval() on the same inode.
An extreme example is:

Locker::eval() wakes an open request(). Server::handle_client_open() starts
a log entry, then call Locker::issue_new_caps(). Locker::issue_new_caps()
calls Locker::eval() and wakes another request. The later request also tries
starting a log entry.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Reviewed-by: Greg Farnum <greg@inktank.com>