cache: improve creation code

cache: improve creation code

Move code to better locations.
Improve test and remove invalid ones
(i.e. no reason to require cache size to be >= then origin).

Correctly comment where the code is doing actual conversion
of other existing volume - we do already a similar thing with
external origins.

Lots of new command line options and combinations is now supported.
Hopefully older syntax still works as well.

lvcreate --cache --cachepool vg/pool  -l1
lvcreate --type cache --cachepool vg/pool  -l1
lvcreate --type cache-pool vg/pool  -l1
lvcreate --type cache-pool --name pool vg  -l1
... and many many more ...