new error code to represent streaming attempts on usupported channels
refactor: outgoing_state simplified
add write{_metadata,,_trailers,end_of_message} implementations
refactor: renamed direction-dependant write_continue operation

Every operation that only makes sense in one direction should be
appropriately be prefixed with "incoming_request" or
"outgoing_response".

Other prefixes that might be used in the future are "outgoing_request"
and "incoming_response".
refactor: write_message -> outgoing_response_write_message

Besides the name change to reflect the fact that can only be used in
outgoing response mode, the arguments were also refactored and
message.start_line is no longer used.

This change is the first in a series to eliminate the need to parse
start_line and make consistent use of the message concept by no longer
requiring "polymorphic" handling. All content-sensitive data is passed
when the context is in use.
refactor: write_metadata -> outgoing_response_write_metadata