pithos: Remove mapfile hexlification functionality

Until now Pithos was using unhexlified mapfiles but now
Archipelago transparently converts those mapfiles
to hexlified.
Mapper functions do not need to convert unhexlified mapfiles
to hexlified ones.
pithos: Differentiate object hashmap from mapfile

Object mapfiles on the storage backend will no more be identified by the
object's hashmap (content depended).
Each object upon creation will be assigned with a unique tag by which they
will be referenced on the storage backend.
This commit contains the necessary modifications in the pithos database.
pithos: Handle properly zero size objects

The backend should not store zero size objects.
This commit contains the necessary modifications for the modular backend.
pithos: Fix copy operation

Newly created objects via copy will be associated with a new mapfile.
Hence, the copy operation should store this mapfile to the storage
backend.
pithos: Extend column list
pithos: Restore sqlite backend

This commit restores the sqlite backend which was broken
by the previous commits.
pithos: Remove prefix for Archipelago volumes

Introduce new property 'is_snapshot' and 'is_available' and
remove old 'archip' prefix for Archipelago volumes.
Refactor relevant functions.
pithos: Extend ObjectWrapper class

Add metadata to ObjectWrapper initialization variables and
propagate 'is_snapshot' property to backend functions.
pithos: Propagate 'is_snapshot' property

Propagate 'is_snapshot' property to modular backend functions.
pithos: Suppress value from modular backend

Suppress 'is_snapshot' return value from modular backend
get_object_hashmap function.
deploy: Update Gunicorn startup conf file
plankton: Pass conf values to Pithos backend

Pass Archipelago config file, XSEG pool size and
mapfile check interval configuration values when
instantiating Pithos backend class.
pithos: Change mapfile check interval default value

Set default value to 1sec instead of 5sec. After taking
a snapshot the user may try immediately to create a new
machine based on the last snapshot. If the mapfile check
interval value is high enough then the end user has to wait
at least the defined interval.
pithos: Expose Archipelago pool to modular backend
pithos: Slight modifications in the backends

* Do not generate/register a mapfile for zero size objects since
there are not stored in the storage backend.
* Several minor fixes
pithos: Fix/improve pithos app tests
pithos: Skip test in case of sqlite backend module
pithos: Remove unused block path and umask variables
plankton: Remove Pithos unused block path variable
pithos: Update backend tests