mysql_cb: extend the backend with the ability to associate the shared networks with the server tags

Similar to what has been done in #676 (closed), we need to extend the existing commands, such as remote-network4-set and other, to interpret the specified server tags and associate the shared networks with these tags.

This ticket covers both DHCPv4 and DHCPv6.

Edited Jun 30, 2019 by Marcin Siodelski

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