chore: Set default negative starting blocks on the example configuration

[jsanmigimeno]

Consider setting default negative starting blocks on the example configuration. This will allow passed SwapUnderwritten events (and similar) to be processed, and will allow expiries to be executed for old underwrites, rather than only being able to process new expiries that are registered from the time at which the Underwriter is initiated. Do note this may cause to have to use archival nodes for the rpcs.

This is a mirror of catalystsystem/generalised-relayer#54