CamelNetworkService: Transition to GTcpConnection.

camel_network_service_connect_sync() now returns a CamelStream with a
base GTcpConnection instead of a CamelTcpStream.  CamelNetworkService
handles peer certificate acceptance during the TLS handshake.

This also adds camel_network_service_starttls() to be called after
issuing a STARTTLS command to a server.  This function replaces the
GTcpConnection with a GTlsConnection in the given CamelStream.

Proxy support is temporarily disabled until I can rework a few things.

New functions:

  camel_network_service_starttls()