Pluggable solutions for automatically persisting last seen `seq`

[Ovyerus]

Provide a behaviour (probably Drinkup.SeqCache) and some default implementations for a system that will cache the last dispatched sequence number, and will automatically resume with it in the event of a restart/disconnect. This saves the user having to manually manage seq separately, and should be less error prone as that is given via a supervision tree arg.

It should probably be done on a throttled timeout (configurable) or something like that, as to prevent unnecessary constant writes and potentially locking up the store location.

Probable default implementations:

• Ecto
• Redis
• Mnesia?