Oleksandr

need to define `toString` method properly for key classes

@alsala @VolodymyrBabaiev @cronicc check description, please.

Related PR: https://github.com/HorizenOfficial/zend_oo/pull/181

@MarcoOl94 since you are updating other links, pls update the one mentioned here as well.

Also review MainchainSynchronizer class

@paolocappelletti is it still a case?