Dataset string memcap 3910/v6

[inashivb]

Link to redmine ticket: https://redmine.openinfosecfoundation.org/issues/3910

Previous PR: https://github.com/OISF/suricata/pull/11105

Changes since v5:

• doc suggestion taken into account
• reorganize commits better so it requires less changes while still making sense logically
• rebased
• minor cleanups whitespaces et al.