mcp: implement a concurrency model for calls

Implement the concurrency model described in #26: notifications are
synchronous, but calls are asynchronous (except for 'initialize'). To
achieve this, implement jsonrpc2.Async(ctx) to signal asynchronous
handling. This is simpler to use than returning ErrAsyncResponse and
calling Respond, and since this is an internal detail we don't need to
worry too much about whether it's idiomatic.

Add tests that verify both features, for both client and server.

Also:
- replace req.ID.IsValid with req.IsCall
- remove the methodHandler type as we can just use MethodHandler

Fixes #26

internal/jsonrpc2/jsonrpc2.go

@@ -22,13 +22,6 @@ var (
 	// If a Handler returns ErrNotHandled, the server replies with
 	// ErrMethodNotFound.
 	ErrNotHandled = errors.New("JSON RPC not handled")
-
-	// ErrAsyncResponse is returned from a handler to indicate it will generate a
-	// response asynchronously.
-	//
-	// ErrAsyncResponse must not be returned for notifications,
-	// which do not receive responses.
-	ErrAsyncResponse = errors.New("JSON RPC asynchronous response")
 )
 
 // Preempter handles messages on a connection before they are queued to the main