Fix possible data-race with isAlive() calls

Fix possible data-race with isAlive() calls

Removed isAlive() calls from functions passed to ThreadPool, and instead use an
unordered_set prior to using ThreadPool to "cache" which entities are not alive.
This is so that if a function passed to ThreadPool modifies the ECManager (e.g.
creating a new entity), then there will be no data race from also calling
isAlive(). (Note that this does not protect against "deleting" entities from the
ECManager during use of ThreadPool. It is expected for the caller to do the
"deleting" after use of stored/called functions is finished.)