Optional ipv6 support

Optional ipv6 support

    This patch introduces a new optional config key 'enable_ipv6'. It
    defaults to false. When true, the 'db_host' config value is parsed
    as an ipv6 address/name. If this fails with nxdomain, then the value
    is parsed as an ipv4 address/name.

    The function inet:getaddr/2 is used to parse the db_host value. As a
    result, driver modules will now receive a parsed IP address tuple
    rather than a hostname or IP address string (even when enable_ipv6 is
    false or unset).

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