Static method for determining the last-known resolver for a domain name.
    Returns the resolver URL on success
    Returns None on error

def get_subdomain_resolver(name, db_path=None, zonefiles_dir=None):
    """
    Static method for determining the last-known resolver for a domain name.
    Returns the resolver URL on success
    Returns None on error
    """
    opts = get_blockstack_opts()
    if not is_subdomains_enabled(opts):
        log.warn("Subdomain support is disabled")
        return None

    if db_path is None:
        db_path = opts['subdomaindb_path']

    if zonefiles_dir is None:
        zonefiles_dir = opts['zonefiles']

    db = SubdomainDB(db_path, zonefiles_dir)
    resolver_url = db.get_domain_resolver(name)

    return resolver_url