Change lock in PullLocalDaemon to only lock on discovery

Summary:
The locks held by read-only pullLocal daemons were causing our discovery instance
to not get the lock and fail at discovery. We don't need to hold the lock while
pulling (only while discovering), so this moves the lock to the appropriate place.

Test Plan: tested in production

Reviewers: jungejason, epriestley, vrana

Reviewed By: jungejason

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2890