Some tests may want to modify system.topology table directly. Add a REST
API to reload the state into memory. An alternative would be restarting
the server, but that's slower and may have other side effects undesired
in the test.
The API can also be called outside tests, it should not have any
observable effects unless the user modifies `system.topology` table
directly (which they should never do, outside perhaps some disaster
recovery scenarios).
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