HTTP transport enhancements

Added HTTP as a transport (accept or connect), abstracting TCP connection from HTTP request-response.
Introduced generic "write close" and "read closed" to grammar (replacing "close request" and "close response").
Reduce HTTP-specific codebase by leveraging more generic concepts (eg. AstReadConfigNode).
Added concept of FlushEvent to the runtime, not exposed in grammar (needed by HTTP connect).
Simplify TCP control protocol, including a version for compatibility checking.
Remove HTTP-based control protocol, to be replaced by same control protocol over BBOSH transport.
Simplify debug output to show expected script, parsed script, script progress and observed script.