storage/blob: Pass blobSize to go-imap-sql

Part of #395 fix

framework/module/blob_store.go

@@ -14,6 +14,8 @@ type Blob interface {
 
 var ErrNoSuchBlob = errors.New("blob_store: no such object")
 
+const UnknownBlobSize int64 = -1
+
 // BlobStore is the interface used by modules providing large binary object
 // storage.
 type BlobStore interface {