Bug#24734971: PERFORMANCE REGRESSION IN DICTIONARY OPERATIONS DUE TO METADATA LOCKING

Problem: Search in, and traversal of lists of MDL_tickets maintaned by
MDL_context becomes time consuming when the number of tickets is
large.

Fix: 1) Add a hash index for the lists to get O(1) lookup based on MDL_key.
     2) Keep track of the subset of the tickets for which
        MDL_context::materialize_fast_path_locks already have been
        called, and only iterate over tickets added since the last
        call.

Change-Id: Ie4275d02ee310112ba36614a9c331da869d71d18
(cherry picked from commit e531c8a23f8edeb4b845be98b5cfa29d46bc7af1)

Author: Dyre Tjeldvoll