topic: call a different wire protocol command to get tns-aware branchmap

To see if remote supports topic namespaces, we announce topics-namespaces
capability and then check it. If it's supported, then we can use a different
branchmap command that gives us branchmap keyed by fqbn. The support for old
"branch:topic" format for the keys is continued through the already existing
8 jobs for topic/default/tns-exchange-2 in 42 minutes and 27 seconds (queued for 1 second)