Because even if it doesn't make a difference in practice, it is
technically more correct to call static methods statically. It
also better advertise that those methods can be used from any context.
This drop a lot of non-core features code and delegate their maintainance
to third parties. Also it open the door to any missing implementation
out of the box, such as Redis for the moment.
Finally this consistently enforce a constraint where there can be one and
only one active cell at any point in time in code. This used to be true for
non-default implementation of cache, but it was not true for default
implementation where all cells were kept in-memory and thus were never
detached from their worksheet and thus were all kept functionnal at any
point in time.
This inconsistency of behavior between in-memory and off-memory could
lead to bugs when changing cache system if the end-user code was badly
written. Now end-user will never be able to write buggy code in the first
place, avoiding future headache when introducing caching.
Closes#3
