Optionalcachedirectory for the decompressed, hash-keyed cache (default: see sigDbCacheDir)
Optionalhashcontent hash to key the cache (avoids reading the source header; supplied from a manifest)
Optionalindexindex to use instead of a sibling .idx (supplied from a manifest so no .idx file need ship)
options controlling where SigDatabase/SigDatabaseSet materialize decompressed caches