pithos: Apply metadata/group limits for the resources

This commit sets a maximum metadata number per Pithos resource
as well as a maximum group number per account.

More specifically:
* Introduces two new settings for Pithos: the maximum number of metadata
each resource (account/container/object) can have and the maximum
number of groups each account can have.
* Adds checks to enforce the above limits.
* Introduces the LimitExceeded exception raised by the backend when these
limits are infringed.
* Refactors the backend implementation of the group/metadata update in
order to reduce interaction with the database.

The affected backend methods are:
* update_account_meta
* update_container_meta
* update_object_meta
* register_object_map
* copy_object
* move_object
* update_object_hashmap
* update_account_groups

Applications, other than Pithos+, that use the Pithos backend
should put a special care on handling these situations.
pithos: Add tests

Test metadata & group limit enforcement.