Make Shard handler threadpool configurable

From profiling of monitor contention, as well as observations of the
95th and 99th response times for nodes that perform distributed search
(or ‟aggregator‟ nodes) it would appear that the HttpShardHandler code
currently does a suboptimal job of managing outgoing shard level
requests.

Presently the code contained within lucene 3.5's SearchHandler and
Lucene trunk / 3x's ShardHandlerFactory create arbitrary threads in
order to service distributed search requests. This is done presently to
limit the size of the threadpool such that it does not consume resources
in deployment configurations that do not use distributed search.

This unfortunately has two impacts on the response time if the node
coordinating the distribution is under high load.

The usage of the MaxConnectionsPerHost configuration option results in
aggressive activity on semaphores within HttpCommons, it has been
observed that the aggregator can have a response time far greater than
that of the searchers. The above monitor contention would appear to
suggest that in some cases its possible for liveness issues to occur and
for simple queries to be starved of resources simply due to a lack of
attention from the viewpoint of context switching.

With, as mentioned above the http commons connection being hotly
contended

The fair, queue based configuration eliminates this, at the cost of
throughput.

This patch aims to make the threadpool largely configurable allowing for
those using solr to choose the throughput vs latency balance they
desire.