Whitespace is now disallowed in metadata keys.

- Strict subset of
  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3442.html
- Useful to avoid unnecessary string copies.

Use of ValueError is too easily misconstrued as an actual defect in the
system under test.

While a full Cartesian product across all variables isn't yet present
this is still reasonably comprehensive.

have a gpr public function which returns cycle clock.

This defect was introduced in 515b0a93.

This will be used for communication of transport-specific values and
objects up to applications.

(1) In grpc._links.service._Kernel.add_ticket, premetadata() the call
if it has not already been premetadataed for any non-None ticket
termination, not just links.Ticket.Termination.COMPLETION.

(2) In grpc.framework.core._reception, add an entry to
_REMOTE_TICKET_TERMINATION_TO_LOCAL_OUTCOME for REMOTE_FAILURE.
REMOTE_FAILURE on a received ticket indicates the remote side of the
operation blaming the local side for operation abortion.

(3) In grpc.framework.core._reception.ReceptionManager._abort, only
abort the operation's other managers if the operation has not already
terminated, as indicated by the "outcome" attribute of the
TerminationManager.

(4) In grpc.framework.core._reception.ReceptionManager._abort, don't
transmit the outcome to the other side of the operation. Either it came
from the other side in the first place and to send it back would be
telling the other side something it already knows, or it arose from a
network failure and there's no confidence that it would reach the other
side.

Rephrase API's such that resolvers are constrained to be able to provide a
default host given just the text of the URI channel target.

This avoids needing to rewrite such details in the core library during
retries, and generally makes things much saner to debug.

This is the public API of the old face package of RPC Framework
extracted into a first-class interface and adapted to metadata, status,
and flow control.