Instrument queries

Starting work on adding instrumentation around every query reaching out to the LDAP server, especially in loops.

This adds `GitHub::Ldap#instrument` that takes an event name and payload with a block that will proxy out to the configured instrumentation service (ActiveSupport::Notifications, for instance). If no instrumentation service is configured, the block is called normally.

Next step is to wrap queries with `instrument`.

cc @rtomayko @sbryant @mcolyer @mikemcquaid @dbussink @jmickeyd 

This is the most substantive change in this branch, which I think is good to go. We're wrapping the chokepoint with instrumentation to get an idea of every search that this library generates. And what this doesn't instrument, https://github.com/github/ruby-net-ldap/pull/1 should provide us with visibility into.