Don't require the interest to be registered, just discard any interest,
this function just sets a bit in a mask, and is not intended to maintain
a balance like the reserve() function is.

When running any session that has Queues which require Resource.CACHE,
check if our loaded estimated size exceeds the quota, and if so;
lock the Resource.CACHE resource exclusively right away and run
an exclusive initial cache size job.

This ensures we cleanup first before doing anything which might
add to the cache at startup time, if deemed needed.

This is a partial fix for issue #737

In order to test when core activities occur by parsing the stderr
in tests, we should make the messages conform more.

At the same time, this restores alignment of columns in core
messages with the element processing related messages.

Also, _scheduler/scheduler.py is updated to make it's activity names
conform to the (current) 5 character limit for the sake of alignment.

The tests/frontend/logging.py test gets it's regexes updated for
the log lines it checks for in stderr.

Cache management fixes

See merge request !1091

The artifact cache emits messages, and we want to allow that in preflight.

We can streamline this call to os.statvfs() in a few places.

This will benefit from a better UtilError being raised, and
and turns the artifact cache's local function into a one liner.

The loop which finds the first existing directory in the
given path has been removed, being meaningless due to the
call to os.makedirs() in ArtifactCache.__init__().

The local function was renamed to _get_cache_volume_size() and
no longer takes any arguments, which is more suitable for the
function as it serves as a testing override surface for
unittest.mock().

The following test cases which use the function to override
the ArtifactCache behavior have been updated to use the new
overridable function name:

   tests/artifactcache/cache_size.py
   tests/artifactcache/expiry.py

A simple object which creates a snapshot of current
usage statistics for easy reporting in the frontend.

A frontend facing API for obtaining usage statistics.

I would have put this on Stream instead, but the Context
seems to be the de facto place for looking up the artifact cache
in general so let's put it here.

This also adds some comments around the main status bar heading
rendering function.

Added some useful status messages when:

  * Calculating a new artifact cache usage size

  * Starting a cleanup

  * Finishing a cleanup

Also enhanced messaging about what was cleaned up so far when
aborting a cleanup.

Updates the known cache size in the main process while the cleanup
process is ongoing, so that the status indicators update live
while the cleanup happens.

This seems to have been copy/pasted from cascache, and
documents the function to possibly return None if defer_prune
was specified, but this function does not expose defer_prune.

Cache management logging enhancements

See merge request !1105

On WSL, os.name and sys.platform will both report "linux". Since the
current Windows-based solution *is* WSL, I see no reason to distinguish
between python from native windows, and python from WSL.

Under WSL, the syscalls don't support binding a unix socket with paths
longer than ~100 characters.

This reverts commit a1fbb22f.