Rename activestore and datastore

Let use configstore by now.

I forgot add the constants. Now all occurences of datastore were removed form the module.

#2: 1510064.configstore.2.patch queued for re-testing.

Core doesn't use "configstore" anymore.
Current terms are bad for UX, they are not immediately obvious.

The terms I see used in core are "active configuration" and "staged configuration", so maybe we should go down that road (giving us labels such as "Write active configuration to staging" and "Import from staging")
Alternatively, perhaps using "database store" and "filesystem store" (or storage) would be obvious enough, but matching core has obvious benefits.

So, something like this perhaps.

Note: the README would need to be rewritten.

Here's some more, seems complete now.
Keep in mind this is completely untested.

#10: 1510064-rename-datastore-9.patch queued for re-testing.

Should fix the tests.