Update opendnssec2 to version 2.1.6.

Upstream changes:

OpenDNSSEC 2.1.6 - 2020-02-11:

* OPENDNSSEC-913: verify database connection upon every use.
* OPENDNSSEC-944: bad display of date of next transition (regression)
* SUPPORT-250: missing signatures on using combined keys (CSK)
* OPENDNSSEC-945: memory leak per command to enforcer.
* OPENDNSSEC-946: unclean enforcer exit in case of certain config
  problems.
* OPENDNSSEC-411: set-policy command to change policy of zone
  (experimental).  Requestes explicit enforce command to take effect.

(he)