BUG/MEDIUM: queue: prevent a backup server from draining the proxy's connections

BUG/MEDIUM: queue: prevent a backup server from draining the proxy's connections

When switching back from a backup to an active server, the backup server
currently continues to drain the proxy's connections, which is a problem
because it's not expected to be able to pick them.

This patch ensures that a backup server will only pick backend connections
if there is no active server and it is the selected backup server or all
backup servers are supposed to be used.

This issue seems to have existed forever, so this fix should be backported
to all stable versions.