Circular dependency in SNMP

[MatteoBiscosi]

In the snmp_dev module, the snmp_cached_dev module is included, however this module reincludes the snmp_dev module itself, causing a possible circular dependency (also calles some functions from the snmp_dev module).
It is needed to create a third module, including all the function using _base_cache_key (snmp_dev_utils.lua) in order to fix all the possible circular dependencies