Make UpdateShardHandler's thread pool configurable

Resource consumption under arbitrary query load can be limited with careful bound on maximumPoolSize in ShardHandlerFactory and appropriate timeouts but it is not possible to do the same for updates because of UpdateShardHandler uses an unbounded cached thread pool. This is a major problem, for example, when trying use SolrCloud as a service and attempting to guarantee SLAs.

I propose to make the UpdateShardHandler's core/max thread pool size and thread keep alive time configurable. If we change the pool size to be bounded, does it make sense to make the queue size also configurable?