Apply Herald subscription effects immediately

Summary:
Fixes T8464. We could incorrectly use a cached value when computing CC's.

Just load a fresh value. There are no other callers that would benefit from this cache, so it's more complicated to reload it correctly prior to publishing than to just skip it.

Also make the PHID headers unique.

Test Plan:

• Verified that users received mail about the transactions which caused them to be added to an object.
• Veirfied that headers no longer have redundant values.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8464

Differential Revision: https://secure.phabricator.com/D13206