Allow tipset worker to work with CSV storage

[frrist]

Describe the bug:

TODO @frrist
Currently, the tipsetwork only operates with a database as the storage output, it should also work with csv files.

Steps to Reproduce:

Lily Version: master

[frrist]

This change will require a new GapFillWorker type to be implemented and some changes to the TipSetWorker. Currently, the TipSetWorker can handle watch, walk, index, and fill tasks. Since Fill tasks require writing to the DB (to mark gaps as filled) the TipSetWorker requires a Postgres database (transactional storage) to operate. We can instead do the following:

1. Modify the TipSetWorker to only handle watch, walk, and index tasks. Removing the need to it to operate with transactional storage
2. Implement a GapFillWorker that strictly handles fill tasks and require transactional storage with it.