Add Streamwise Catchup mode

[bartelink]

equinox-sync and Propulsion.EventStore originally had a mode in which, when syncing from EventStore to Cosmos, one could:

1. start projecting from the tail without first doing a full sync operation
2. request N stream-readers (defaults to 1)
3. have each event observed on a stream that's not currently in sync request a ReadStreamEventsForwardAsync-based catchup read and project in order to enable the event to be written to the target correctly.

Large amounts of the code are in place for this with TODO markers etc; work has been deferred as its not anticipated that we'll use this mode (reading streamwise is extremely resource-intensive on an EventStore server relative to reading the $all stream, and the feature is only relevant where the wall clock time involved in an initial sync would render it prohibitive to do so)