server: only suggest a max of one pack per receive-objects cycle.

server: only suggest a max of one pack per receive-objects cycle.

Since the client only handles one at a time and forgets the others anyway,
suggesting others is a bit of a waste of time... and because of the cheating
way we figure out which index to suggest when using a midx, suggesting packs
is more expensive than it should be anyway.

The "correct" fix in the long term will be to make the client accept
multiple suggestions at once, plus make midx files a little smarter about
figuring out which pack is the one that needs to be suggested.  But in the
meantime, this makes things a little nicer: there are fewer confusing log
messages from the server, and a lot less disk grinding related to looking
into which pack to suggest, followed by finding out that we've already
suggested that pack anyway.