Characteristics of pre-Upgrade request

[martinthomson]

The following attributes - which are normally attributed to HTTP/2.0 requests - are not available for the pre-Upgrade request in HTTP/1.1. These require defaults:

• Stream ID (default to 1)
• Priority (default to 0.5/2^30)

We also need to determine the status of the stream, which probably needs to be half-closed from the server side.

[mnot]

Discussed at SF Interim; Martin to incorporate stream ID 1; priority 0.

[mnot]

HTTP/1.1 request must be complete before client starts sending HTTP/2. This will block further requests on the connection until the upgrade is effective.