“A counsel, attorney, or solicitor is neither bound nor at liberty to divulge the secrets of the cause with which he may have become confidentially intrusted, nor can official persons be called upon to disclose any matter of State, the publication of which may be prejudicial to the community; but this is the only privilege allowed by the law in this respect; and clergymen, medical men, friends, servants, and others are bound to tell such secrets (being facts relevant to the issue) with which they may have become by whatever means acquainted.”