Several improvements for the stateless streamable mode, plus support for
a 'distributed' (or rather, distributable) version of the stateless
server.
- Add a 'Stateless' option to StreamableHTTPOptions and
StreamableServerTransport, which controls stateless behavior.
GetSessionID may still return a non-empty session ID.
- Audit validation of stateless mode to allow requests with a session
id. Propagate this session ID to the temporary connection.
- Peek at requests to allow 'initialize' requests to go through to the
session, so that version negotiation can occur (FIXME: add tests).
Fixes#284
For #148
98e67fdf62