Refactor queueing-related classes in presto-main module to a new presto-dispatcher module

[prithvip]

In order to support work related to prestodb/rfcs#23, one of the work items is to re-factor the presto-main module to better separate code related to the queueing and execution phases. This is a large refactor that will involve multiple PRs, hence this issue to track it.

At a high-level, we want to re-factor presto-main into a presto-dispatcher and a presto-main module, with queueing related classes in presto-dispatcher, and everything else in presto-main. It is likely that we will also need to create a third module, presto-main-common, for classes that are needed for both queueing and execution.