Rather than loading them lazily. This allows us to remove all
the yield statements and spurious arguments for the get_next
methods.
It also allows us to replace all instances of get_next_txn with
get_next since get_next no longer needs to access the db.
This is for setting up dependencies that require work on startup. This
is useful for the DataStore that wants to read a bunch from the database
before initiliazing.
Handle the fact that events can be persisted out of order, and so to get
the "current max" stream token becomes non trivial - as we need to make
sure that *all* stream tokens less than the current max have also
successfully been persisted.
