⚡️ zb: Pipeline client-side handshake

We send all that we can for each auth at the same time and also receive
all server reponses in the same go. This reduces latency and hence
improves performance.

Unfortunately, cookie auth requires a round-trip to the server to get
the cookie context first so we can't fully pipeline that one.

The server side is not possible to pipeline as it the server just
responds to commands from the client and it can not assume that clients
will always pipeline all commands. We will however, simplify the server
handshake code in a following commit.

Fixes dbus2#493.