Several tests were manually running migrations in a large copy-paste blob of code. This makes the test less useful as it's possible to break the code while keeping the tests green by introducing a new migration that never gets run in the tests, and it's also difficult to reason about what the tests are doing.
A new test helper library is introduced with a TestMigrationLoader that can both run Flyway migrations, or load specific migrations in the cases a specific set of migrations need to be loaded. Existing tests are migrated to use the new code.
The EC_DOMAIN_LINK MariaDB table stores links between domains. This is problematic, as both updating and querying this table is very slow in relation to how small the data is (~10 GB). This slowness is largely caused by the database enforcing ACID guarantees we don't particularly need.
This changeset replaces the EC_DOMAIN_LINK table with a file in each index node containing 32 bit integer pairs corresponding to links between two domains. This file is loaded in memory in each node, and can be queried via the Query Service.
A migration step is needed before this file is created in each node. Until that happens, the actual data is loaded from the EC_DOMAIN_LINK table, but accessed as though it was a file.
The changeset also migrates/renames the links.db file to documents.db to avoid naming confusion between the two.
