Store shortest paths with observations in a db

[Zannick]

Basically, expand on #190 with more storage capacity, probably by using another rocksdb.

[Zannick]

Once this all works, we ought to have some background cleaning up (potentially when we trim solutions and such). Mainly I'm concerned that we may have routes that are not useful to keep around, such as trie entries whose routes are too long (longer than the time limit is extremely unlikely) or entries where the requirements are stricter than another entry with a better time.