Protect DB Contract from remote call

[thiagonobrega]

Make the DB contracts function that perform data update (e.g., insert, update and delete) operations accessible by the proper contracts. For example, the PidDB save must be called only PidService.

At the moment we have to :

• design the security mechanism
• implement the mechanism for the AuthorityDB
• implement the mechanism for the ExternalPidDB
• implement the mechanism for the UrlDB