Problem: fd leak in tweetnacl with one ctx per thread

Solution: add a crypto [de-]initialiser, refcounted and serialised
through critical sections.
This is necessary as utility APIs such as zmq_curve_keypair also
call into the sodium/tweetnacl libraries and need the initialisation
outside of the zmq context.
Also the libsodium documentation explicitly says that sodium_init
must not be called concurrently from multiple threads, which could
have happened until now. Also the randombytes_close function does
not appear to be thread safe either.
This change guarantees that the library is initialised only once at
any given time across the whole program.
Fixes #2632

src/ctx.hpp

@@ -1,5 +1,5 @@
 /*
-    Copyright (c) 2007-2016 Contributors as noted in the AUTHORS file
+    Copyright (c) 2007-2017 Contributors as noted in the AUTHORS file
 
     This file is part of libzmq, the ZeroMQ core engine in C++.
 
@@ -233,8 +233,6 @@ namespace zmq
         int vmci_family;
         mutex_t vmci_sync;
 #endif
-
-        mutex_t crypto_sync;
     };
 
 }