Handbook/Storage: Various improvements

• Minor grammar updates
• Add tip about replicating using ZFS snapshots
• List popular backup utilities
• When backing up, keep a list prime packages
• Update sample CARP configuration
• Add tip about sharing highly available storage across the network

Reviewed by: emaste, jrm, Pau Amma <pauamma@gundo.com>
Differential Revision: https://reviews.freebsd.org/D43948