Stop using Mutex_m
Stop using Mutex_m

It adds weird stuff to the backtrace, which is annoying, see #89.

pigoz/spring,rails/spring,maclover7/spring,slobodankovacevic/spring,oggy/spring,smathy/spring,ypresto/spring,mariusbutuc/spring,oggy/spring,y-yagi/spring,grosser/spring,universsky/spring-1