Add camel_stream_set_base_stream().

Replaces the GIOStream passed to camel_stream_new().  The new base
stream should wrap the original base stream, such as when adding
Transport Layer Security after issuing a STARTTLS command.
Add camel_stream_ref_base_stream().

Replaces camel_stream_get_base_stream().

Now that the base stream is replacable, this needs to be thread-safe.
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()
Remove unused CamelTcpStreamSSL.
Remove unused CamelTcpStreamRaw.
Remove unused CamelTcpStream.
Remove unused camel_getnameinfo().