CBL-8475: Use AtomicRetained in Replicator::Subreplicators

[snej]

Fixes a race condition when a Replicator is torn down.

Some code had to be altered, because AtomicRetained is a bit harder to use: you have to first call get() to get a temporary Retained object, then use that.

[github-actions]

This is a release branch and commits are restricted.

Please confirm this PR is one of the following:

• A response to a customer ask
• A change per our security policy
• A non-functional change (i.e. changes needed for building an older version)
• A change that has been granted an exception (please comment)

[borrrden]

The downstream stuff needs to go into fleece release/4.1 before it can go into lite-core release/4.1

[snej]

Ubuntu build failed with a GCC error building Fleece tests.

couchbase-lite-core/vendor/fleece/Tests/SupportTests.cc:476:15: error: alias template 'AtomicRef' requires template arguments; argument deduction only allowed for class templates
  476 |     AtomicRef r = new AtomicRetainedTest();

Why wasn't this caught by Fleece's CI? ... hm, one difference is that Fleece's Ubuntu builds use GCC, while LiteCore's use Clang. But Clang on macOS didn't get this error. Different versions of Clang, perhaps?

[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	64.6
functions	77.4
instantiations	71.03
lines	75.97
regions	72.28