Previously, we'd send one interface_version event for "ei_handshake"
immediately but all others after the client requests handshake.finish.
This was too confusing to document and not clear how it would work, so
let's make this simpler by splitting it up.
There is now a handshake_version event from the server, sent immediately
on connection that denotes the maximum version number for the interface.
And a handshake_version request from the client which must be the first
one by the client.
This tests the protocol layer which is hard to test using libei/libeis.
Similar to the generated C bindings we compile a eiproto.py file that is
then used in the test to talk protocol directly to the eis-demo-server
that we start up.
By sending the specific messages and checking things happen as we expect
on the socket we can verify that the EIS implementation is correct (and
robust enough).
In theory this could also be used to test some other binary with an EIS
implementation and the scaffolding is there to set LIBEI_TEST_SERVER to
that binary. Wether this works is untested though...
