multiple session legs support

[bobzilladev]

Adding support for multiple session legs.

This adds two configuration options:

• WithAdditionalServers(addrs []string)
• WithMultiLeg(enable bool)

If multi-leg is enabled, then the ConnectAddresses in the Auth response of the first session are used to determine what additional sessions should be set up. The ReconnectingSession will accumulate more sessions on connect, then return as normal. ReconnectionSession will handle fan-out of calls to the underlying tunnel_client.Session as before, except now there are multiple of them.

If the first connection is made using least-latency, it will lock in to that ConnectAddress so reconnects will go back to the same region. Heartbeat callbacks are done for each underlying session, including appropriate region and latency information.

[jrobsonchase]

Makes sense to me! Some public API nits, but overall looks good 👍

[jrobsonchase]