databases/couchdb2: new port via PR#218844

Summary: JSON document database with HTTP API & scalable multi-master sync

A number of people provided signficant assistance while developing this
port. Thank-you for your perseverance and assistance.

PR: 218844
Submitted by: dch
Reviewed by: Reshad Patuck, Richard Gallamore, Tobias Herre, mat, ler, pi
Approved by: jrm (mentor)
Differential Revision: https://reviews.freebsd.org/D16819