* new proc to foster different size retention policy implementations
The new proc, decreaseDatabaseSize, will have different implementations
per each driver. For example, in future commits we will implement a size
retention policy thanks to partitions management, in Postgres.
* RetentionPolicy: use of new instead of init for ref object types
* waku_archive: fix signatures in decreaseDatabaseSize methods
* retention_policy_size: minor cleanup of comments and imports
Ivan FB
Hanno Cornelius