Replace "web" and "conduit" magic session strings with constants

Summary: Ref T4310. Ref T3720. We use bare strings to refer to session types in several places right now; use constants instead.

Test Plan: grep; logged out; logged in; ran Conduit commands.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4310, T3720

Differential Revision: https://secure.phabricator.com/D7963
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
Remove `metamta.send-immediately`

Summary:
Ref T3857.

  - Always send mail via daemons. This lets us get rid of this config, and is generally much more performant.
  - After D7964, we warn if daemons aren't running.

Test Plan: Sent some mail.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3857

Differential Revision: https://secure.phabricator.com/D7965