system_keyspaces: implement load_dc_rack_info
system_keyspaces: implement load_dc_rack_info

To implement that, we will resort to a cache mechanism, instead of doing the
query all the time.  This is mainly because we want to avoid overfuturization
of the callers, that are usually just interested in passing simple strings
around.

We will be able to intercept all updates to it, and maintain consistency with our
internal cache. The updates are not done in this patchset.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

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