Interface entity.aggregate into transactions

Transaction state management

Connect transactions to entity-core with EntityTxnEvents
implementation of TxnEvents

A lock manager. Threads may lock values, waiting to obtain
the lock if one is already outstanding. Locks are re-entrant
and only released when fully unwound or by force.

If a lock is currently held by another thread, the current
thread may wait until the lock becomes available.

Protocols and multimethods for interfacing transaction
events to actions.