Support for interrupting transactions via the transactional REST endpoint

o Ensure ThreadToStmCtxBridge.getTopLevelTransactionBoundToThisThread() does not throw when interrupted [FIX]
o Rename assertInTransaction to assertInUninterruptedTransaction all over the place
o Changes TransactionRegistry: Now uses an ActiveTransaction marker per tx instead of a singleton
(needed to hold a reference to the tx such that we can interrupt it from a different thread)
o Adds number of transaction interrupts to TransactionMonitor

Signed-off-by: Andres Taylor <andres.taylor@neotechnology.com>