The NoteDb migration writes many loose objects to All-Users, since this
is the simplest way to write data to All-Users with high parallelism.
However, this results in poor performance of the All-Users repo
immediately after migration, for example taking tens of seconds to load
each account on GerritForge's server.
Work around this by running GC immediately after the migration process,
both offline and online. This is not perfect, because it is not
represented by a NotesMigrationState enum value, meaning the GC will not
be resumed if the server is uncleanly shut down during the process. (As
with all GCs, this should not result in repo corruption.)
Change-Id: Iffc89e1c125ef5db57298daccabd7e1fe4af8d27
8f1dc546c1