rtmp: Add support for adobe authentication

This is mostly used to authenticate the client when publishing.
Tested with wowza and akamai.

Some but not all servers support resending a new connect invoke
within the same connection, so always reconnect for sending a new
connection attempt. This matches what other applications do as well.

The authentication scheme is structurally pretty similar to http
digest authentication, but uses base64 instead of hex strings.

Signed-off-by: Martin Storsjö <martin@martin.st>
