Fixed incorrect validation that forced ExecutorID uniqueness.
Fixed incorrect validation that forced ExecutorID uniqueness.

In the ReregisterSlaveMessage message validation, we were
requiring the ExecutorID to be unique across a whole agent.
In fact, only the {FrameworkID, ExecutorID} tuple must be
unique.

This closes #248

andschwa/mesos,dforsyth/mesos,reneploetz/mesos,gsantovena/mesos,kaysoky/mesos,asamerh4/mesos,jpeach/mesos,chhsia0/mesos,asamerh4/mesos,gsantovena/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,jpeach/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,kaysoky/mesos,gsantovena/mesos,kaysoky/mesos,asamerh4/mesos,kaysoky/mesos,andschwa/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,asamerh4/mesos,kaysoky/mesos,gsantovena/mesos,andschwa/mesos,Gilbert88/mesos,andschwa/mesos,dforsyth/mesos,reneploetz/mesos,asamerh4/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos,abudnik/mesos,asamerh4/mesos,gsantovena/mesos,abudnik/mesos,reneploetz/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,Gilbert88/mesos,chhsia0/mesos,asamerh4/mesos,andschwa/mesos,reneploetz/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,jpeach/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,andschwa/mesos,dforsyth/mesos,andschwa/mesos,abudnik/mesos,dforsyth/mesos,jpeach/mesos,jpeach/mesos