Improve caching of current domain

[colemanw]

Overview

The current domain is read dozens, sometimes hundreds of times per request, so it needs to have solid caching. This consolidates 4 different ways it was being cached (or in the case of api3, not cached) into one central spot.

Before

Current domain cached during bootstrap but never flushed in the event of changes.
Several other methods of reading the domain were used that bypassed that cache or used some other cache.

After

All centralized to one cache, which is automatically flushed by hook whenever updates are made (which is rare but important to respond to, and essential for unit tests)

[civibot]

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[demeritcowboy]

Gave the code a read-over and poked around a bit with a single-domain english-single-language setup. So not a full thumbs up but so far so good.

The postSave took me a minute but I'm reading it as not adding a new hook, but implementing the hook within core itself.

[seamuslee001]

I believe we have plenty of test coverage here that if something was to break it should show up in the test coverage merging

[colemanw]

The postSave took me a minute but I'm reading it as not adding a new hook, but implementing the hook within core itself.

Yes @demeritcowboy the way to implement a hook callback internally is by adding a listener.