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.

This reverts commit 48d833a9, reversing
changes made to 66e9d8e8.

application handle the authentication.

We suffered a bug whereby doing a follow-up write to another write could
resurrect a deleted stream, causing all sorts of crash.

Fix: when a stream becomes writable (vs when we start writing) take a
ref on the stream, and only relinquish it once we're done writing.