Improved concurrency and thread-safeness in Mailbox

[fbacchella]

Simplified Mailbox, always thread safe, so MailboxSafe usages are removed and the class is marked at deprecated.

[trevorbernard]

Since Sockets are known to be to be not thread safe, what is the purpose if making internals thread safe? It might the case we're adding extra work for no material gain.

[trevorbernard]

This PR satisfies all c4 requirements so I must merge but it's worth considering my question. I do not know the answer.

[fbacchella]

Mailbox was using YPipe, a class that is really complicated to understand and ensure to be correct. It this class could be totally removed, it would be really better for reliability of jeromq. I’m also investigating #905, and especialy #905 (comment) and I’m chasing down many occurrence of non compliance of java memory model, where a non-protected or non volatile variable could be access with wrong values.

The original author was not totally sure of thread usage, as he added some lock. Either they are to be removed, or the class made thread safe. I choose the way that open the most usage.

MailboxSafe was handling concurrency in a custom way, using a standard class should improve reliability too.