seastar/rpc: add fmt::ostream_formatter<> for rpc::connection_id
seastar/rpc: add fmt::ostream_formatter<> for rpc::connection_id

this is a follow-up of 2975b61264694cb4ce229ca82f0d9b553bef0d76,
the fmt::formatter for rpc::connection_id is used by
connection::get_stream(), but this function is not compiled by
our regular CI test, so it was missed.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

Closes #1162

scylladb/seastar,scylladb/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,syuu1228/seastar