Improve stress tests

Summary:
* Disable secondary index lookup verification because these
cause deadlocks and innodb spends most of its time in deadlock
rather than executing queries.

* Disable toggling of innodb_prefix_index_cluster_optimization
which is not ported yet.

* Add ability for checksum of the table computed in a separate thread
while the table is modified by bunch of threads.  This is useful for
testing certain features under concurancy.

Test Plan: mtr

Reviewers: jtolmer

Reviewed By: jtolmer