Optionalcachedirectory for the decompressed, hash-keyed cache (default: see sigDbCacheDir)
Optionalexcludeload every shard except these ids (e.g. ['full-top','full-rest'] for a current-only view)
Optionalhashcontent hash to key the cache (avoids reading the source header; supplied from a manifest)
Optionalincludeonly load these shard ids (e.g. ['base-current','current-top']); omit to load all
Optionalindexindex to use instead of a sibling .idx (supplied from a manifest so no .idx file need ship)
options for SigDatabaseSet.openManifest -- the base cache options plus per-shard enable/disable