CRM-20455 - Add specialized event for queries

CRM-20455 - Add specialized event for queries

Overview
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The recently merged PR for CRM-20455, https://github.com/civicrm/civicrm-packages/pull/180, uses the
GenericHookEvent for modifying all SQL queries.  GHE is flexible, but it
comes with a slight performance penalty (juggling a few internal arrays).
The penalty is usually too small to care if you're firing one hook. However, with CRM-20455,
we can foresee the event firing hundreds or even thousands of times within a given page-view.

This PR adds a smaller/simpler class for use with `modifyQuery`. There will be a different PR to use it.

Before
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* Don't have `QueryEvent`

After
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* Do have `QueryEvent`