Modify Compactor so that it does not check for compactions every ms

[dlmarion]

Put some backoff in the loop when checking for compactions. Incrementally wait longer between checks, maybe up to 3 seconds.

[dlmarion]

Added Compactor.getWaitTimeBetweenCompactionChecks which currently waits 3 seconds between checks for the next compaction job. @keith-turner was talking about a different implementation based on the number of Compactors.

[keith-turner]

I was thinking of making the sleep time a function of the # of compactors. Hoping to achieve something where on average a few compactors check in with the coordinator every second. Thinking this can be done by increasing each compactors sleep time as the total number of compactors increases. I'll take a crack at this.

[keith-turner]

@dlmarion can you review the changes in 022f4b9

I tested this locally. I started and stopped compactors and saw the trace message reflect the number of compactors.

[dlmarion]

LGTM