internal/jsonrpc2: cleanup reply handling
internal/jsonrpc2: cleanup reply handling

Instead of having a Parallel method, we only have Reply, which must also be used
for Notify messages (with a nil response).
This has no real functional impact but makes it easier to refactor in the next
cl.

Change-Id: Ifd4316dd71706de7913c69e6be539b966800e9dd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/226837
Run-TryBot: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools