Cache the storm id to executors mapping on master to avoid repeat computation

Now nimbus will collect all the topologies's conf/topology-ser/storm-base to compute in a scheduling round, which is a very heavy work. The scheduling will still take to minutes even we now change to RPC heartbeats and assignment distribution.

So i decide to redesign the scheduler, so we can only schedule the topologies that need to: that have dead workers or not enough number workers.

Here i checkout out the code and found that the id->executors mapping is computed every time for every topology, which is really a heavy computation and totally not that necessary, because this mapping is fixed invariable for a topology unless we rebalance or kill it.

So i refactor the code a little here, and this is more powerful after the scheduler is resigned for delta-scheduling[ which is very lightweight even there are thousands of topologies on one cluster.]

For now this is enough for us.