BUG-650: speedup getSubtransaction()

getSubtransaction() is on fast path and it performs two map lookups:
first with containsKey() and then with get(). We know the map cannot
contain null values, so we can perform only the get() and then make
a precondition check on the returned value being non-null.

Change-Id: I477969c8512e3c22e5b7ae5455e0281613a9c752
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-650: speedup getSubtransaction()"