This adds ability to specify 'named' threadpool to searches.

Named threadpools needs to be configured in yml explicitly .

For eg:

threadpool:
     fred:
        type: fixed
        size: 30

[ Here fred , named threadpool is configured with default size of 30]

SearchRequestBuilder and ScrollRequestBuilder takes threadpool name ,
but defaults to SEARCH. REST API accepts new http parameter 'threadpool'
which can take name of threadpool to run queries.