Various fixes for compliance with HTTP/2 spec

Motivation:

A few items were identified where the http2 codec is out of compliance
with the spec.

Modifications:

- Fixed handling of priority weight on the wire. Now adding 1 after
reading from the wire and subtracing 1 before writing.

- Fixed handling of next stream ID. Client streamIds were starting at 3,
but they need to start at 1 to allow the upgrade from HTTP/1.1. Also
making next stream ID logic more flexible. Allowing the next created
stream to be any number in the sequence following the previously created
stream.

- Disallowing SETTINGS frames with ENABLE_PUSH specified for server
endpoints. This means that attempts to write this frame from a server,
or read it from a client will fail.

Result:

The http2 implementation will be more inline with the spec.