purge_all() and purge_by_definition() look in the configuration
for which caches are available and then creates them to purge them.
The configuration stores the values used by initialise(), not
initialise_unit_test_instance() and would therefore fail to purge
all caches if they were not purged by another means.
In the case of filestore, it's purged by unit tests, in the case
of memcache(d), it purges the whole store when a single definition
is requested.
Therefore all configuration was moved into the configuration file
during unit tests and does not have any special override codes in
the unit test infrastructure.
If the cache does all the dereferencing when it stores and loads
the objects, then the cache loaders don't need to do that work.
This is true of all caches that use something other than PHP's memory
to store their results.
By holding onto a reference of the global $CFG object we
can be sure that it isn't destroyed before we are done
with it.
This allows for caches to be used within destructors for
purposes such as delayed writes.
This change is a large change to the way sessions are handled
within MUC after it was discovered that session did not function
as expected when any store other than the default session store
was being used.
As part of this change the session loader has been largely
customised in order to consolidate session data for the loader.
The unit tests have also being greatly increased to provide
better coverage for sessions.
Rajesh Taneja