Make daemons ignore "Unreachable" commits and avoid duplicate work

Summary:
Ref T9028. This improves the daemon behavior for unreachable commits. There is still no way for commits to become marked unreachable on their own.

• When a daemon encounters an unreachable commit, fail permanently.
• When we revive a commit, queue new daemons to process it (since some of the daemons might have failed permanently the first time around).
• Before doing a step on a commit, check if the step has already been done and skip it if it has. This can't happen normally, but will soon be possible if a commit is repeatedly deleted and revived very quickly.
• Steps queued with bin/repository reparse ... still execute normally.

Test Plan:

• Used bin/repository reparse to run every step, verified they all mark the commit with the proper flag.
• Faked the reparse exception in the "skip step" code, used repository reparse to skip every step.
• Marked a commit as unreachable, ran discover, saw daemons queue for it.
• Ran daemons with bin/worker execute --id ..., saw them all skip + queue the next step.
• Marked a commit as unreachable, ran bin/repository reparse on it, got permanent failures immediately for each step.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16131