[SEARCH] Execute search reduce phase on the search threadpool

Reduce Phases can be expensive and some of them like the aggregations
reduce phase might even execute a one-off call via an internal client
that might cause a deadlock due to execution on the network thread
that is needed to handle the one-off call. This commit dispatches
the reduce phase to the search threadpool to ensure we don't wait
for the current thread to be available.

Closes #7623