[SEARCH] close active contexts on SearchService#close()

When we close a node all pending / active search requests need to be
cleared otherwise a node will wait up to 30 sec for shutdown sicne there
could be open scroll requests. This behavior was introduces in 1.5 such that
versions <= 1.4.x are not affected.

Closes #8940

Conflicts:
	src/main/java/org/elasticsearch/search/SearchService.java