Flag activity on a bucket before finding a vbucket.

This errs on the side of dirty, but is still racy.
Small buffer on the periodically registrar.

Work blocks registration, so if work *causes* registration, or
peripheral work holds a lock that that something doing background work
wants a lock held by something wanting to register causes a
mini-deadlock.

Really, a buffer size of one *should* be enough to prevent this from
happening because unrelated things should work themselves out, but
more won't hurt.  It's also possible that the right combination of
concurrent jobs could do something similar, but a given periodic
dispatcher should only be running one type of job once at a time per
unit of registration (e.g. storage backend) which should generally not
be competing for the same locks.

I do need some more verification that this doesn't leave any data
unwritten, but I'm going to call this issue closed for now.

cbugg: close bug-509