Improve sender ratchets deletion schedule

[mulmarta]

Currently we have a constant const MAX_RATCHET_BACK_HISTORY: u32 = 1024 which is used for two things: we keep X = 1024 past ratchets for a sender and we throw an error if we get a message more than Y = 1024 ratchets in the future.

We need something more flexible:

• allow applications to configure Y with the option to make Y "infinite" meaning that we always accept a message (we still keep X past ratchets from the most recent message)
• allow applications to configure X. Consider the option to let the StorageProvider deal with ratchet deletion by adding a function prune_ratchets(generation) on PriorEpoch that deletes ratchets older than generation