Convert OperationCompleter to OperationLimiter

The completer and limiter functions are related to each other, so
encapsulate them in a single object.

Since a TransactionProxy cannot really do anything without touching the
limiter, make sure we instantiate it in constructor, preventing some
volatile reads in the fast path.

Change-Id: I4cf31ef46c11676611a62db7a794f504712ce5af
Signed-off-by: Robert Varga <rovarga@cisco.com>