Have BaseBackend inherit from ArgBasedSingleton

Have the base pithos+ backend inherit from
synnefo.lib.singleton.ArgBasedSingleton.

This makes repeated attempts to re-instantiate any pithos+ backend with
the same arguments reuse the pre-existing backend object.

snf-pithos-backend/pithos/backends/base.py

@@ -35,6 +35,7 @@
 DEFAULT_QUOTA = 0 # No quota.
 DEFAULT_VERSIONING = 'auto'
 
+from synnefo.lib.singleton import ArgBasedSingleton
 
 class NotAllowedError(Exception):
     pass
@@ -60,7 +61,7 @@ class ItemNotExists(NameError):
 class VersionNotExists(IndexError):
     pass
 
-class BaseBackend(object):
+class BaseBackend(ArgBasedSingleton):
     """Abstract backend class that serves as a reference for actual implementations.
     
     The purpose of the backend is to provide the necessary functions for handling data

snf-pithos-backend/setup.py

@@ -65,7 +65,7 @@ CLASSIFIERS = []
 
 # Package requirements
 INSTALL_REQUIRES = [
-    'snf-common>0.9.13',
+    'snf-common>0.10.0',
     'SQLAlchemy==0.6.3',
     'alembic>=0.3.4, <0.4',
 ]