Add timed lockguard support

Add support for recording mutex wait and held times; but *only* if
either time exceeds a given threshold.

Identifying mutex contention (where one thr

Add timed lockguard support

Add support for recording mutex wait and held times; but *only* if
either time exceeds a given threshold.

Identifying mutex contention (where one thread is waiting a long time)
is extremely useful in tracking down performance issues, however the
median times are typically fast, and so if we logged events for all
mutex locks we'd quickly consume the whole trace log. Instead, if we
only record trace events for locks which are slower than some limit,
we can discount much of the "noise" and focus on the signal.

This patch also renames "lock.acquire" to "lock.wait" as IMHO that's a
clearer (and more concise) name for that the span represents.

Note that care must still be taken in using these macros, as each one
requires 3x calls to steady_clock::now(); even if no trace_event is
recorded. As such, consider adding to a non-default category, and only
enabling on-demand.

Change-Id: I4884cd8a2e4381ab16fb9ecfab0478546d52f0ef
Reviewed-on: http://review.couchbase.org/93889
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Build Bot <build@couchbase.com>

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Use Complete event type for scoped events

Use the new Complete trace type to implement the scoped (TRACE_EVENT*
/ TRACE_FUNCTION*) event macros. This reduces the number of TraceEvent

Use Complete event type for scoped events

Use the new Complete trace type to implement the scoped (TRACE_EVENT*
/ TRACE_FUNCTION*) event macros. This reduces the number of TraceEvent
objects needed for these events by half; well worth the modest (1
word) increase in size to support Complete events (needed to add 64bit
duration field).

Implemented by adding a RAII-style ScopedEventGuard class; which
captures the arguments for a trace event, and if enabled logs the
event in it's destructor.

Note this change required the removal of the TRACE_EVENT() (varargs)
macro; as we now need to capture the types of the args (as the call to
logEvent() happens at the end of the scope).

Change-Id: I975927385616887a26ea8f894a1d06c08e322ecf
Reviewed-on: http://review.couchbase.org/93879
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Tim Bradgate <tim.bradgate@couchbase.com>

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