Allow a HarbormasterBuildMessage to be sent to any object

Summary:
See T13054. This prepares for Buildables to be sent messages ("attach", "done scheduling builds") to fix races between Harbormaster and Differential.

The buildTargetPHID is replaced with a recipientPHID in the API. An additional change will fix the storage.

In the future, this table could probably also replace HarbormasterBuildCommand now, which is approximately the same bus, but for Builds.

Test Plan: Viewed builds with messages. Sent messages with harbormaster.sendmessage. Processed messages with bin/phd debug task.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D19062