Split backend/database between server/dump-manager.

Created by: efritz

The purpose this PR is to move all requirements of a persistent filesystem into the dump-manager process.

The resulting APIs are minimally changed but not very nice. These will be reworked in a future PR to minimize data transfer and coupling.

Digest of actions:

• server/backend/database.ts copied to dump-manager/backend/database.ts (github does not show this as a rename)
• sever/backend/database.ts modified so that all public methods perform an HTTP GET to the dump-manager
• added API routes for each public method of the database class in dump-manager
• purgeOldDumps and cleanFailedUploads tasks moved from server/tasks.ts to dump-manager/tasks.ts (they select files to purge)
• added an "internal" router in server to answer data needed by purgeOldDumps cleanFailedUploads tasks (which require db lookups to determine upload state)