Allow creation of multiple sessions per connection

[russjones]

Purpose

The in-memory forwarding server was incorrectly creating a "session" channel request for every connection upon accepting a client connection. The in-memory forwarding server has been updated to request a "session" channel when the client issues the request.

Implementation

• Move the creation of the srv.ServerContext to when either the session or direct-tcpip channel request is being handled. This allows the creation of multiple sessions.
• Moved connection closers out of srv.ServerContext and added them into forward.Server and are closed calling the Close method.
• Cleaned up comments to bring them inline with current Teleport standards.

Related Issues

Fixes #1811