Improve library thread-safety

[JamesWrigley]

This library is a pretty thin wrapper around ZMQ and as such exposes most of its semantics, including its lack of threadsafety around sockets. What with Julia 1.12 coming up I think we should consider adding a threadsafe API for Socket.

I also noticed a case of ZMQ.jl being thread-unsafe in close(::Context):

ZMQ.jl/src/context.jl

Lines 74 to 82 in b4f30ae

	function Base.close(ctx::Context)
	if isopen(ctx) # don't close twice!
	for w in getfield(ctx, :sockets)
	s = w.value
	if s isa Socket && isopen(s)
	s.linger = 0 # allow socket to shut down immediately
	close(s)
	end
	end

The function will first set the linger time of all sockets to 0 and close them, and then terminate the context. This is not threadsafe because the sockets may be running on a separate thread (e.g. the IJulia heartbeat thread).