Added Wt benchmark

Here's an implementation of the tests for Wt, as requested in issue #92

A few notes:

Wt::Dbo does not support working without transactions. Because using transactions gives
an unfair disadvantage to Wt, I decided to keep one transaction live per thread,
for all of the tests that only do reads. In the updates test, I'm using a new transaction
for every fetch and update, as per the requirements. This might make Wt unusually slow in
the updates test. The use of transactions in that test actually causes MySQL to report
deadlock errors, so we have retry the transaction sometimes.

A workaround is implemented for the PostgreSQL issue #783. Preferably I'd like to have that
feature resolved. Let me know if you will resolve this issue, as it will require removal of
this workaround.

Is this implementation ok? If so, could it please be included in the benchmark suite?