ITS#9584 Track refreshing status explicitly

A refresh can jump across multiple threads, we cannot just lock a
mutex, we need to track who that is and clear when finished.

In addition to that, fix our internal syncrepl session tracking pulling
it closer to RFC 4533, refreshDone now represents the receipt of
refreshDone flag. Refreshing status is maintained for plain refreshes
(and times when we might be starting one but don't know for sure).

We still reschedule a new sync with a delay if there is another one
running but tracking refreshes this way paves the way to being able to
wake them up if we start tracking them somehow.
3 jobs for !438 with its9584 in 54 minutes and 51 seconds (queued for 1 minute)
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Status Name Job ID Coverage
  Build
passed build-gnutls-mit-standalone-lloadd #10107

00:47:46

passed build-no-threads-no-slapd #10105

00:03:47

passed build-openssl-heimdal-lloadd #10106

00:53:38