Now servers can send messages/bundles to another server, and answer to a
server on the same sock...

also use send_message instead of raw sockets in test_server

since the client has access to the socket, it can lookup the port name
using getsockname()[1]

also refactoring of tests

use struct members directly, also replace an unpack + slice calls with
an unpack_from one

also rename tests/parser.py to tests/test_paser.py, so it's discovered
by pytest