Dynamically increasing backtracking window

[patriknw]

It can be difficult to configure the optimal akka.persistence.r2dbc.query.backtracking.window. If too large there is overhead when starting up since all envelopes (without event payload) must be loaded. If too small there is a risk of hitting "Rejecting unexpected sequence number" error, especially when combined with scaling number of projection instances.

Note that this must also play well with akka.projection.r2dbc.offset-store.time-window.

One idea is to be able to increase the backtracking window in case of these errors, so that next restart look further back.