applier: name ballot_watcher fiber properly

Currently all ballot_watcher fibers for different appliers have
identical names: "applier_ballot_watcher". It's hard to distinguish
them in logs and to know which ballot_watcher belongs to which applier,
and that complicates debugging quite a bit.

Let's fix this and name the fibers properly. With master's uri at the
end.

NO_DOC=no behaviour changed
NO_CHANGELOG=no behaviour changed
NO_TEST=doesn't need one