This is a breaking change. This commit separates syncstorage and tokenserver settings into separate structs that are contained by a parent `Settings` struct. This means that any env vars that hold settings specific to syncstorage (e.g. `SYNC_DATABASE_URL`) have been renamed to `SYNC_SYNCSTORAGE__DATABASE_URL`. Any settings that were moved from the top-level `Settings` struct to the lower level, syncstorage-specific struct will now have a `SYNC_SYNCSTORAGE__` prefix instead of a `SYNC_` prefix.
Closes#1276
- always prefix primary key names with their table name (id -> bso_id,
collection_id). a best practice in spanner, as when tables
interleave, their shared id names must always match
- bso -> bsos (every other table name's plural)
- give batches an expiry index
Closes#313
