Fallback to the filesystem metadata if something is not found in the database

While running the registry in mirrored mode, which will happen during the initial testing stages, it would be ideal to make it read first from the database and fallback to the filesystem if not found there. Currently, we're reading from the database first and halting if not found.

Api GET Operations

• blobHandler !291 (merged)
• catalogHandler (Falling back not possible)
• manifestHandler !292 (merged)
• tagsHandler (Falling back not possible)

Api PUT Operations

• schema 1 manifests !301 (merged)
• schema 2 manifests !295 (merged)
• oci image manifests !296 (merged)
• oci image indexes !336 (merged)
• manifest lists !336 (merged)
• tagging manifests (No need to fallback)
• cross-repository blob-mounting !307 (merged)

API DELETE Operations

• blobs !298 (merged)
• blobuploads (No need to fallback)
• manifests !311 (merged)
• tags !310 (merged)