For GRPCOperation's, ensure finish _handler can only be called once, and release it when called, so weak ptrs needn't be used with it, and the call won't be released until the finish handler is called.
When the connectivityMonitor determines the connection has been lost, pull the host disconnect call. Creates an unreliable connection when connectivity is restored. Calling finishWithError: is sufficient.

Revert "Revert "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()""

Move C-layer call creation into GRPCChannel, so that it always acts on a
non-destroyed channel.

Listen for connectivity in the GRPCCall, and destroy the channel.

Revert "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()"

Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll().

Because the category makes the protocol apply to all `NSMutableDictionaries`, it can't assume keys are `NSString`. Because these types are covariant, the change is not breaking.

TODO:
- Documentation
- Make public
- Check I’ve implemented all NSMutDict required methods

The GRPCCall import must be in the header because it defines a protocol
that GRPCRequestHeaders uses.

The GRPCCall.h file is already correctly imported in the
GRPCRequestHeaders.m implementation file. This header shouldn’t need to
import GRPCCall and should specify a class forward declaration for
reference.