Add purge undo and write-back dirty buffers option to MariaDB TPROC-C

[sm-shaw]

When running a sequence of TPROC-C workloads against MariaDB, purging activity from previous tests can impact the performance of subsequent tests, according to when the purge happens.

Adding an option to purge undo and write back dirty buffers after a test is complete will enable a sequence of tests to be more deterministic.