sstate: Add eventhandler which cleans up stale recipe data

"Incremental builds do not work well when renaming recipes or changing
architecture" is a long standing issue which causes people considerable
pain. We've struggled for a long time to come up with a way to
generically address the problem.

There are additional issues where removal of a layer caused data to
continue to exist and additionally, changing DISTRO_FEATURES also caused
problems in an existing TMPDIR.

This patch attempts to address this by adding a mapping between stamp
files and manifests. After parsing we can easily tell which stamp files
are still reachable, if any manifest has a stamp that can no longer be
reached, we can remove it. Since this code ties this to the sstate
architecture list, it will not remove data from other than the current
MACHINE (and its active architectures). It does not clean the sstate
cache so if another build activates something which was cleaned, it
should reinstall from sstate.

We can also go one step further, depending on the setting of
SSTATE_PRUNE_OBSOLETEWORKDIR, workdirs which are no longer active can
also be removed. This avoids the buildup of many old copies of data in
WORKDIR for example when versions are upgraded.

The one thing which may surprise people with this change is if you
remove a layer, data added by that layer will be "uninstalled" before
the next build continues. I believe this is a feature and a good thing
to do though.

This code is safe with existing builds. If something isn't in the new
index it simply isn't removed. Since changes to the sstate code trigger
a rebuild, after this merges, we can assume the code will start to
detect changes from that point onwards.

[YOCTO #4102]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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