Don't pre-load dbxrefs in LoadUtil unless asked

Disable pre-loading of the dbxref cache by default as it slows down normal
curation - especially session creation.  Also if cache isn't preloaded,
remember to check for existing dbxref before creating another.

See [curation-tool:#751]
Suppress warnings about using given()
Warn if trying to upgrade from wrong db version
Add unique index on db_id+accession, inc. version

We should have incremented the schema version in [a460bcdb2].
Update to version v784