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