Consider using a database for pool accounts mappings management on PEP server

[andreaceccanti]

Currently the Argus PEP server relies on the gridmap dir as the "database" for pool account mappings.
When replicating the pep-server, the gridmap dir must be shared across NFS, and this can become a bootleneck for pep-server scalability in large sites.

The code that manages the pool account mapping should be changed to allow for a different backend for storing pool account mappings, e.g. REDIS, which maintains information in memory but can be configured to periodically persist the database on disk.