A DDL on table would trigger change in view metadata update.
We acquire MDL locks on view and the dependent tables during
this process. We release these acquired locks just after
finishing the view metadata update, and we do not hold the
MDL lock until the statement is committed.

An expectation from binary log subsystem is that it assumes
all MDL acquired by the statement are held till commit
phase, so two statements acquiring conflicting sets of locks
can't be committed at the same time. This assumption is
broken by release-before-commit optimization done during
view metadata update.

This patch enables MDL locks acquired during view metadata
update to be held until commit. A test case is added to make
sure we acquire lock after view metadata is updated. The test
case for Bug#25685371 is removed as we no more release locks
at the end of view metadata update, and the problem reported
there cannot occur.

Change-Id: I3757428fc01788b9314a961b5fd2f83830811485