Add basic handshake negotiation

[alecgibson]

This change updates how the client and server communicate when first
connecting, in a non-breaking way.

Current behaviour

• client creates and connects to a socket
• client constructs a Connection object with the socket
• the backend binds to the socket and constructs an Agent
• the Agent sends an init message down the socket when constructed
• the Connection receives the init message and initialises itself
  using information in the message

Motivation for change

There are currently a few things that would be nice to "negotiate" when
a client connects. For example, clients may wish to:

• the same src value when reconnecting, so that it persists as a
  stable "session ID" (which has been implemented here)
• set their default type
• establish the exact versions of OT types being used
• establish plugins and versions to use

More detail can be found here: #337

New behaviour

• client creates and connects to a socket
• client constructs a Connection object with the socket
• the backend binds to the socket and constructs an Agent
• the Agent sends an init message down the socket when constructed
• the Connection checks the init message to see if the backend is
  capable of handshaking
• if the backend cannot handshake (ie it's old), then the client
  initialises itself as before
• new behaviour starts here
• if the backend can handshake, then it will disregard the init
  message, and send its own hs "handshake" message to the server
• the backend receives the handshake message and sends a response
• the Connection receives the new hs message and initialises itself
  using information in the message (like before, but with more or
  different information information)

Other changes

As a convenience for some tests, which had to be tweaked (because they
assumed the connection was established immediately), this change also
adds an optional callback parameter to Backend.connect, which will
be invoked once the connection handshake completes.

We also add a guard for arbitrarily large seq, which will no longer be
reset on reconnection, so might hit integer overflow on particularly
long-lived Connection objects.

[coveralls]

Coverage increased (+0.07%) to 96.851% when pulling 4b3ff13 on handshake into 7f9d98f on master.