Backup the table user_lastaccess which holds data for when a user last accessed a course.
And also restore to the table user_lastaccess from the .xml file created in the backup.

As part of the tests preparations, some backups are created, but
the tests aren't calling to their ->destroy() method at the end.

The destroy() method is needed to go cleaning and unsetting various
objects that, only then, will free resources, like memory, open files...

This isn't critical under Unix OSs... but Windows is really sensible
and keeps all open files locked, so any write operation on them fails.

- Some small namespace/use/MOODLE_INTERNAL/requires reorganization.
- Move file phpdocs to class phpdocs.
- Move external method names to be new "execute" default ones.
- Move unit tests to matching core_grades\external namespace.
- Add missing clean_returnvalue() calls.

This adds a new webservice that creates gradecategories as a batch,
and deprecates the old single creation call, which is superseded.
It also patches a few small issues in the single WS, for any integrations
currently relying on that webservice.

Some recent tests do set a date time element
to ##now## or tomorrow and, immediately after that
the look if, effectively, ##now## and #tomorrow#
have been set (with minutes resolution).

Problem is that, between the field is set and the field
is verified, it can happen that the time advances to
next minute (from H:M:59 to H:M+1:00) and then the
assertion fails.

To avoid this, we could have lowered the resolution to be
hours... but that doesn't solve the problem just makes it
to happen less often.

So, instead of that... we are setting the 2 now and tomorrow
cases to be "today noon" and "tomorrow noon" (12:00:00) so
we ensure they won't be ever in the risk of jumping of minute.

AMOS BEGIN
 CPY [grade,core],[gradenoun,core]
 CPY [grade,core],[gradeverb,core]
AMOS END