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