R1:db2425b300fb

Do initial repository imports at a lower priority and finish importing commits before starting new ones

Summary:
Fixes T11677. This makes two minor adjustments to the repository import daemons:

- The first step ("Message") now queues at a slightly-lower-than-default (for already-imported repositories) or very-low (for newly importing repositories) priority level.
- The other steps now queue at "default" priority level. This is actually what they already did, but without this change their behavior…