sysutils/zap: Update to version 0.6.8.

Release information: https://github.com/Jehops/zap/releases/tag/0.6.8

Port changes:

• Remove warnings in pkg-message about option changes in older pre-releases.
• Update pkg-message to mention blog post describing a replication strategy.

Reported by: David Samms <dsamms@nw-ds.com>
Approved by: swills (mentor, implicit)