MINOR: connection: rearrange conn_get_src/dst to be a bit more extensible

MINOR: connection: rearrange conn_get_src/dst to be a bit more extensible

We'll want conn_get_src/dst to support other means of retrieving these
respective IP addresses, but the functions as they're designed are a bit
too restrictive for now.

This patch arranges them to have a default error fallback allowing to
test different mechanisms. In addition we now make sure the underlying
protocol is of type stream before calling the family's get_src/dst as
it makes no sense to do that on dgram sockets for example.