ability to ask a specific tracker for more peers

[morph]

Hello, this bug was originally submitted at: ​https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=509043

With a multi-tracker torrent, it would be nice to be able to ask specific
trackers for more peers.

Also, does the "ask tracker for more peers" selection get grayed out when
*any* of the trackers dont want you to announce yet, when *none* of the
trackers want you to announce yet, or when the *first* tracker doesn't want
you to announce yet?

A lot of the rest of the interface is mono-tracker centric (just filed two
other bugs on this), so if this button gets grayed out only when the first
tracker wants you to leave it alone, this may be another multi-tracker bug
in transmission.