Merge DHCPv6 global parameters fetched from the CB into the configuration

SrvConfig::merge must be updated to merge global parameters into existing staging or current configuration.

After conferring with Marcin, we agreed to expand the scope of this issue to incorporate merging:

1. globals
2. options defs
3. options
4. shared-networks
5. subnets

As this work is largely replicating what was done for v4.