New ListKeys API in syncstorage

New ListKeys API in syncstorage

The ListKeys API returns all keys in the given namespace matching
key search pattern.

Also update sdltester -tool to test ListKeys api:
  listkeys
    - List keys in the given namespace matching key search pattern.

Upgraded ci/Dockerfile golang image version tag to 1.16. This is
due to expired Root CA certificates in golang:1.12 image which
result an error when building docker image:
  fatal: unable to access 'https://gopkg.in/tomb.v1/': server
  certificate verification failed. CAfile: none CRLfile: none

Version: 0.8.0

Issue-Id: RIC-110
Signed-off-by: Petri Ovaska <petri.ovaska@nokia.com>
Change-Id: Ic02a3f127934a1e02e01a36c573d035db34dbd6f