Tablet throttler: post-v18 changes

[shlomi-noach]

Followup to #13341

v18 RC1 is released, and the following should be done for v19:

• v18 added gRPC inter-checks, with fallback to HTTP for backwards compatibility. v19 should drop that compatibility and completely remove HTTP checks.
• This calls for a refactor of the throttler, removing all InstanceKey references, any hostname:port use. All access should be done by tablet alias or tablet reference.
• A long time coming, while the throttler was tested in the past releases via endtoend tests (and extensively so), it did not enjoy proper unit/race tests. These should now be added.
• A few other minor updates to the throttler code are in place, such as adapting to recent golang atomic types.