caa_get_cycles: caa_ prefix for type, use CLOCK_MONOTONIC

clock_gettime() of CLOCK_MONOTONIC mimicks better the behavior of a
cycle counter, since it is not affected by NTP major time updates.

Also, now prefix "cycles_t" with a caa_ prefix, so it does not clash
with application namespaces. Anyway, caa_get_cycles() is mostly used in
testing.

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