Setting the newly added compression_level field of
grpc_op::send_initial_metadata by a server now has the effect of
applying that compression level for the subsequent call messages leaving
the server. The ultimate meaning of the level depends on the client's
supported compression algorithms.
from the peer. This both complies with the RFC and ensures that if the
user sets a higher limit than the 16K default, we won't incorrectly
reject data sent before settings are exchanged.
Also fix proxy tests.