Adding query to temporary buffer to achieve higher update rate and NRT

Currently Luwak have at least two issues that slows down subscriptions update rate. One of them is read write lock and the second is building terms white list. If we want to achieve NRT it is almost impossible when there are many subscribed queries.

That change adds temporary "uncommited" collection of queries. After exceeding the threshold all updates are flushed and openSearcher method starts.