Refactor imapsql ExternalStore to use modules

Closes #303

framework/module/blob_store.go

@@ -0,0 +1,30 @@
+package module
+
+import (
+	"errors"
+	"io"
+)
+
+type Blob interface {
+	Sync() error
+	io.Reader
+	io.Writer
+	io.Closer
+}
+
+var ErrNoSuchBlob = errors.New("blob_store: no such object")
+
+// BlobStore is the interface used by modules providing large binary object
+// storage.
+type BlobStore interface {
+	Create(key string) (Blob, error)
+
+	// Open returns the reader for the object specified by
+	// passed key.
+	//
+	// If no such object exists - ErrNoSuchBlob is returned.
+	Open(key string) (io.ReadCloser, error)
+
+	// Delete removes a set of keys from store. Non-existent keys are ignored.
+	Delete(keys []string) error
+}