Re-introduce sync vs unsync enablers

Re-introduce sync vs unsync enablers

Eliminate iteration over unmodified enablers when synchronizing the
enablers vs event state.

The intent is to turn a O(m*n) algorithm (m = number of enablers, n =
number of event probes) into a O(n) when enabling many additional events
when tracing is active.

This change is done both for event enablers and for event notifier
enablers.

[ Re-introduce commit after integration of Trace Hit Counters. ]

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I6a6ce09b10a98f281e03fab3a69b851a44bd8657