- Had to chnage the core API to add a destroy function pointer in
  grpc_auth_metadata_processor.
- Tested end to end.
- Fixed some issues in the server_auth_filter (we were not checking the
  length which put us at risk of an overflow).

(1) Move metadata and details constants for gRPC-on-the-wire tests into
grpc.test_common.

(2) Drop definitions of setUpModule and tearDownModule from a unit test
module that, because it uses the load_tests protocol, never had those
methods called anyway. :-(

(1) Call "cancel" on each future, not on the list of futures.

(2) If and when futures mature their actions should simply abort all
outstanding operations and cancel any other futures. They should not
shut down the _End's internal thread pool; only the termination action
of the last operation to terminate should shut down the pool (in the
case of their having been active operations at the time at which the
_End's stop(grace) method was called).

'stream' may be left nullptr after a pop, so don't try to dereference
it.

(via nlewycky@google.com).

cc grpc/grpc-common#274, #2896

- Removing service_accounts credentials. These credentials just have
  drawbacks compared to service_account_jwt_access credentials, notably
  in terms for security.
- Renaming Google specific credentials with a Google prefix for C and
  C++. This should be done as well for wrapped languages.