Multiple user level locks per connection

This is to merge the patch that implements multiple user level locks per connection contributed by Konstantin Osipov. Description from commit comments:

"
GET_LOCK() function in MySQL allows a connection to hold at most
one user level lock. Taking a new lock automatically releases the
old lock, if any.

The limit of one lock per session existed since early versions
of MySQL didn't have a deadlock detector for SQL locks.
MDL patches in MySQL 5.5 added a deadlock detector,
so starting from 5.5 it became possible to take multiple locks
in any order -- a deadlock, should it occur, would be detected
and an error returned to the client which closed the wait chain.

This is exactly what is done in this patch: ULLs are moved
to use MDL subsystem.
"

See also http://kostja-osipov.livejournal.com/46410.html