More interop

[edsko]

After this PR, the situation is as follows:

grapesy server versus official reference client

Test	Python	C++	Go	Java
cancel_after_begin	✅	✅	✅	✅
cancel_after_first_response	✅	✅	✅	✅
client_compressed_streaming	❔	✅	❔	✅
client_compressed_unary	❔	✅	❔	✅
client_streaming	✅	✅	✅	✅
custom_metadata	✅	✅	✅	✅
empty_stream	✅	✅	✅	✅
empty_unary	✅	✅	✅	✅
large_unary	✅	✅	✅	✅
ping_pong	✅	✅	✅	✅
server_compressed_streaming	❔	✅	❔	✅
server_compressed_unary	❔	✅	❔	✅
server_streaming	✅	✅	✅	✅
special_status_message	✅	❔	✅	✅
status_code_and_message	✅	✅	✅	✅
timeout_on_sleeping_server	✅	✅	✅	✅
unimplemented_method	✅	✅	✅	✅
unimplemented_service	✅	✅	✅	✅

where ❔ are test cases that are not supported by the reference client.