Added new header grpc-payload-bin
Added new channel arg for setting max payload size
Ability to create a GET request in client filter
Ability to parse the payload from header in server filter.

Also make the script fetch protoc from the usual place.

- Propagation of (rpc) method name.
- Invocation of the hook at (call, channel) x (creation, destruction)
- Added enum to identify the source of invocation.
- Fixed testing. Went from test fixture to simple test.

much metadata and refuses to send responses with too much metadata.