Moved Apc_target::destroy(), Apc_target::enable() and Apc_targe::disable()
definitions to my_apc.h so that they can be inlined.

Apc_targe::disable() now calls Apc_target::process_apc_requests() only if
there're APC requests. This saves one pthread_mutex_lock() call.

Overhead change:
Apc_target::disable              0.04% -> out of radar
Apc_target::enable               0.03% -> out of radar
Apc_target::process_apc_requests 0.02% -> out of radar
pthread_mutex_lock               0.43% -> 0.42%
pthread_mutex_unlock             0.26% -> 0.25%