Allow overriding of socket handling methods

This is primarily needed for ClientAuthHandler, which does not want
to throw error messages to the outside during protocol detection.
Implement protocol detection

This introduces a new initial handshake for negotiating the supported
protocols and connection features on both sides. It is completely
backwards compatible with older releases, in which case we go into
fallback mode.

Arguably, we could've used a nice, verbose, text-based handshake, but
that would've introduced questions around string formats, parsing and so
on. Also I felt like doing some bitbanging, so now the probing only
exchanges a few bytes which are described in the wiki [1]. If we ever
plan to use a more verbose or different format, changing the magic that
starts the whole shebang will be sufficient to indicate a new format.

Immediately after probing, if both core and client support the new format
(and a protocol other than the legacy one),we'll enable SSL and compression [2]
as appropriate, instead of doing it somewhat later in the middle of the legacy
handshake.

To retain compatibility, the magic number sent by the client is designed such
that older cores will immediately close the connection; we'll then reconnect
in compatibility mode. The other way round, if an older client connects to
a new core, we'll figure out that there's no magic being sent and switch to
legacy mode as well.

The unchanged legacy protocol is also the last resort even if both ends speak
the new handshake as long as we don't have an alternative to offer. So for now,
we'll probe for protocol support, get back the legacy protocol as only choice,
and use that as before. This also disables early SSL and compression mentioned above.

This means that 3rd party clients could already implement the handshake in
preparation for the future without changing anything else. Note that they should
also implement the detection of older cores in order to stay compatible with them -
simply detect a disconnect after the first few bytes sent and reconnect again
in compat mode.

[1] http://bugs.quassel-irc.org/projects/quassel-irc/wiki/Doc_quassel_protocols
[2] Not implemented yet.