Warn if daemons are not running

Summary:
Currently, we try to mostly-kind-of-work if daemons aren't running (for example, we send mail in-process). I want to stop doing this. A major motivator is that metamta.send-immediately is confusing for a lot of users and frequently the cause of performance problems. Increasingly, functionality of applications depends on the daemons (Harbormaster, Drydock, Nuance all require daemons to do anything at all). They're also fairly stable/robust/well-tested and no reasonable install should be running without them.

This will let us simplify or remove some flags (like metamta.send-immediately) and simplify some other processes like search indexing.

Test Plan: Stopped daemons, loaded warnings, saw daemon warning. Started daemons, reloade, no warning.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3857

Differential Revision: https://secure.phabricator.com/D7964