add ProtocolVersion to ReattachConfig (hashicorp#171)

* add ProtocolVersion to ReattachConfig

A client connecting to a plugin process needs to know the resolved ProtocolVersion, so this PR adds that information to the returned reattach config. Prior to this, client.NegotiatedVersion() always returned 0.

* Add a test for correct protocol version in reattach config

Merge pull request hashicorp#161 from hashicorp/jbardin/stopping-recv…

…-loop

grpc stream errors are terminal

add SyncStdio

Merge pull request hashicorp#140 from hashicorp/manual-listener

ServeConfig supports a manual listener

Merge pull request hashicorp#139 from marefr/fix_stdio_backward_compa…

…tibility

Handle stdio service not implemented

Merge pull request hashicorp#138 from hashicorp/reflection

Register grpc reflection service

update grpc

Fix test broken by 9d4515e (hashicorp#122)