Move domain links outside of the MariaDB database

[vlofgren]

Updating and querying this table is very slow and bogs down both the loader and ranking calculations by a substantial amount. The actual data itself is pretty small, 75,000,000 x 16 bytes = 1.2 GB in prod, but all the indices and uniqueness constraints blows this up to ~10 GB in MariaDB. If we split the data across nodes this would be a few hundred megabytes each, and we'd shave off hours of processing.

Sketch of a solution:

• Keep it in memory in the index service and have some API for querying it (maybe via QS)
• Loader writes to file to a file which gets switched over atomically as a whole batch by the index svc

[vlofgren]

We also probably want to rename the LinkDB component to something else, to avoid confusion, since it's not really links but documents. To do this we need a migration step to rename the file...

[vlofgren]

Implemented as part of #69