Design: Maintaining retention window asynchronously in the DB

[Shaptic]

There is no db reingest equivalent in Soroban RPC. We should discuss the extent to which we want to support managing gaps in the database due to downtime or other inconsistencies. Options include:

• trimming the database of everything that is not continuous on startup
• erroring/forcing reingestion of any gaps in the retention window
• allowing users to reingest arbitrary spans of history and/or to find/fill gaps in existing history like Horizon
• doing nothing and just forcing users to wait for the retention window to fill before they get continuous history
• others?

Each of these will result in a different level of work so we should decide first.

[sydneynotthecity]

Objective is to write a one-pager outlining design choices and implementation details to maintain db retention window. Implementing the work is out of scope.

[mollykarcher]

Closing as a dup of #196