fix(mutex): insufficient lock location info logged
Before this patch, when an attempt to lock a mutex failed, only the previous lock location was printed. But we still had no idea where that fail occured.
With this patch the log message looks as follows:
can't lock mutex at src/traft/node.rs:234:14,
already locked at src/traft/node.rs:876:44
Also, this patch makes the corresponding test more strict.