Delegate MonitorMixin#synchronize body to Monitor#synchronize.
It makes guarantee interrupt safe (because Monitor#synchronize is
written in C). I thought Ruby implementation is also safe, but I
got stuck failure <http://ci.rvm.jp/results/trunk_test@P895/2327639>
so that I introduce this fix to guarantee interrupt safe.
Recent monitor.rb has performance problem because of interrupt
handlers. 'Monitor#synchronize' is frequently used primitive
so the performance of this method is important.
This patch rewrite 'monitor.rb' with 'monitor.so' (C-extension)
and make it faster. See [Feature #16255] for details.
Monitor class objects are normal object which include MonitorMixin.
This patch introduce a Monitor class which is implemented on C
and MonitorMixin uses Monitor object as re-entrant (recursive)
Mutex. This technique improve performance because we don't need
to care atomicity and we don't need accesses to instance variables
any more on Monitor class.
