Event comparison optimization

[bestchai]

Right now, when two executions are compared, events in two processes with the same host are compared one-by-one. That is, each event in each process is compared to each event in the other process to see if they have matching event strings. This algorithm can be vastly improved with the implementation of a hash-table so that look-up is just O(1)

[Issue created by pattypcw: 2015-03-05]