net/kea: upgrade 1.5.0 -> 1.6.0

• submitter becomes maintainer

PR: 240399
Submitted by: Andrey Pevnev <apevnev@me.com>
Approved by: hrs (maintainer)
MFH: 2019Q3
Relnotes: https://downloads.isc.org/isc/kea/1.6.0/Kea160ReleaseNotes.txt
Security: CVE-2019-6472, CVE-2019-6473, CVE-2019-6474