mcp: improvements for 'stateless' streamable servers; 'distributed' mode

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

mcp/transport.go

@@ -40,8 +40,8 @@ type Transport interface {
 type Connection interface {
 	// Read reads the next message to process off the connection.
 	//
-	// Read need not be safe for concurrent use: Read is called in a
-	// concurrency-safe manner by the JSON-RPC library.
+	// Connections must allow Read to be called concurrently with Close. In
+	// particular, calling Close should unblock a Read waiting for input.
 	Read(context.Context) (jsonrpc.Message, error)
 
 	// Write writes a new message to the connection.