SES-1719 MemoryStore delays obtaining exclusive write until first actual
write operation is invoked.
SES-1719 native store now delays locking until first write operation in
transaction.
SES-1719 check if transaction was initialized before releasing lock
SES-1719 minor fixes in transaction init flag setting.
Merge branch 'master' into ses-1719
SES-1719 memory store transaction locking updated to allow concurrent
reads while no modification operation has started.
SES-1719 brought Native store locking in sync with approach in Memory
Store. In addition removed hard check in begin() for active state of
transaction - unnecessarily harsh to throw an exception, begin() can
simply block instead.
Merged in ses-1719 (pull request #65)

SES-1719 improve locking behavior for transactions in memory and native store
SES-1719 fix in memory store rollback/commitInternal to only invoke
store-specific internal rollback/commit when modifications have been
made (otherwise the store throws a NPE or assertionerror.