refactor: go straight to PeerCache for fetching topology
There is no need to go via Topology. Fetching does not access or
modify the Topology state, and none of the functions on Topology
update the PeerCache. So there is no point serialising the fetching
with RebuildRoutes (or anything else).

This does mean the connections' receiveTCP goroutines will take a
direct lock on PeerCache, but a) it is only a read lock, and b) they
already take a write lock in another code path.

There may be some extra load on the PeerCache as a result of this
change, due to the topology fetching of multiple connections no longer
being funneled through the single Topology goroutine. Consequently,
write locks on the PeerCache may take a little longer to
acquire. Until we encounter this as a genuine problem though it's
better to stick with the simpler code.