[blob-service] Move database module to database/old

Summary:
Restructure modules a bit to make it easier to add new database interface.

Moved database mod into database/old and extracted database/error module
because it will be used in the new model too.

Test Plan: cargo build

Reviewers: michal, jon, varun, patryk

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8440