datasets: parse defaults section from yaml

datasets: parse defaults section from yaml

Datasets can now have a global defaults setting in suricata.yaml. In
case the settings for memcap and hashsize are not find in the yaml or
rule, this shall be the fallback.

Example:

datasets:
  defaults:
    memcap: 100mb
    hashsize: 2048
  ua-seen:
    type: string
    load: datasets.csv