feat(persist): clean old entries upon loading store file

[gsanchezgavier]

Adds a garbage collect mechanism to remove entries from the storer file saved on disk which has been expired. Expiration is calculated using the same TTL that is being used to remove storer files and the mechanism remove the entires that has not been updated in a period greater than the TTL

Fixes #294

Context for the reviewer:

• Storer files are created one per instance of the integration running on the host, since a hash, computed from the args, is added to the file name.
• Entries on the storer file are identified by the metric set attributes, usually entity attributes.
• The current mechanism removes the hole file when it is not updated.
• The implemented mechanism remove entries that are not updated.