COMMIT COULD NOT BE COMPLETED"

Problem:
========
When a SQL thread which is waiting for commit lock is killed
and restarted it causes a transaction to be skipped on slave.

Analysis:
========
when SQL thread is at a state where a DML is waiting for MDL
commit lock if SQL thread is killed then position are getting
updated in memory. i.e in the existing design positions are
flushed before the actual commit because of this rli object
will have its positions updated but the transaction is yet
to be committed.  When the SQL thread is restarted it reads
position from the rli object and hence the last transaction
gets skipped on slave.

Fix:
===
When SQL thread is killed at a stage where it is waiting for
commit lock, the commit fails and an error is reported back
saying "Commit could not be completed and Query execution
was interrupted".  As part of fix SQL threads positions that
existed before the commit are persisted and they are
restored back on error.

Similar symptoms exist in case of MTS as well. In MTS
"The slave coordinator and worker threads are stopped,
possibly leaving data in inconsistent state" error is
reported. In MTS a bitmap is maintained for successful
commits. This bit map is cleared on error and the old
positions are retrieved from the checkpoint which points to
old positions.