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)

Signed-off-by: Jakub Wieczorek <jakub.wieczorek@neotechnology.com>