maintner: split Corpus.PollLoop into Corpus.Sync + Corpus.SyncLoop

This also adds a maintnerd flag --sync-and-quit.

And unexports some internal guts.

Change-Id: I4f2a491d830550576b285a2a7df6c89cc93c2312
Reviewed-on: https://go-review.googlesource.com/38384
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
4 files changed