There were some minor references to start-matching-loop in the tests.

BUG: the match-oder for market orders contained lvl-ref (same side as
the order) instead of lvl-mtc (opposite side), so that market orders
could be matched against limit orders of the same side!

Since orders are now matched upon insertion, we needed to modify some
tests.

We introduced performance issues due to software transactional memory.

This fixes issue #16.