Remove ambiguity about systemd service files being separate.
Add URL for more information about systemd service files.
Add note about public keys being mandatory for self-signed certs.
Make cert/key file naming consistent throughout document.

Improve documentation for artifact cache installation

See merge request !777

Daniel Silverstone authored Sep 04, 2018 and Tristan Maat committed Sep 04, 2018

The garbage collector can take too long to get around to cleaning
up the Queue and Process instances in completed Job instances.  As
such, FDs tend to leak and in very large projects this can result
in running out of FDs before a build, fetch, track, or other process
can complete.  This patch reduces the chance of that by only creating
the queue when it's needed, and forcing the queue and process instances
to be deleted when the parent is finished with them.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

jobs.py: Reduce FD leaks from queues and process objects

See merge request !778

For symmetry with _create_tree.

Track number of bytes required to commit or pull an artifact and return
this data on pull() and commit(). This takes deduplication into account.

The ArtifactCache.commit() return value is more accurate for what
__artifact_size is being used for, as it takes the deduplication done by
the CAS cache into account.

Before this commit, the cache size was being calculated every time an
artifact was committed to or pulled to the artifact cache. This is a
very IO demanding operation that doesn't scale as the cache gets bigger.

Now that the commit() and pull() methods of the ArtifactCache return the
number of bytes added, we can rework the code to calculate the size of
The cache only once at startup and to dynamically update it afterwards.

The ArtifactClean.clean() is also changed to return the number of bytes
removed instead of the new size of the cache, so that we no longer need
the ArtifactClean.clean._set_cache_size() method.

The CacheSize job is no longer used and is removed.

Fixes #573.