'Get the most recent association that has been set for this
server URL and handle.
str -> NoneType or Association'

def txn_getAssociation(self, server_url, handle=None):

    if (handle is not None):
        self.db_get_assoc(server_url, handle)
    else:
        self.db_get_assocs(server_url)
    rows = self.cur.fetchall()
    if (len(rows) == 0):
        return None
    else:
        associations = []
        for values in rows:
            assoc = Association(*values)
            assoc.secret = self.blobDecode(assoc.secret)
            if (assoc.getExpiresIn() == 0):
                self.txn_removeAssociation(server_url, assoc.handle)
            else:
                associations.append((assoc.issued, assoc))
        if associations:
            associations.sort()
            return associations[(-1)][1]
        else:
            return None
