Set socket buffers before establishing TCP connection 

The window scale option carried in SYN segment is computed from socket's
receive buffer size. So we need to set this buffer size before calling
connect or bind.

Problem: working with binary keys is painful

Minor code refactoring

Use generic session class whenever possible

The use of binary for CURVE keys is painful; you cannot easily copy
these in e.g. email, or use them directly in source code. There are
various encoding possibilities. Base16 and Base64 are not optimal.
Ascii85 is not safe for source (it generates quotes and escapes).

So, I've designed a new Base85 encoding, Z85, which is safe to use
in code and elsewhere, and I've modified libzmq to use this where
it also uses binary keys (in get/setsockopt).

Very simply, if you use a 32-byte value, it's Base256 (binary),
and if you use a 40-byte value, it's Base85 (Z85).

I've put the Z85 codec into z85_codec.hpp, it's not elegant C++
but it is minimal and it works. Feel free to rewrap as a real class
if this annoys you.

Problem: mechanisms are lagging behind specs

RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.

Another ZMQ_STREAM simplification

Simplify ZMQ_STREAM socket implementation

Problem: ROUTER+RAW was not a clean API

- designed for TCP clients and servers
- added HTTP client / server example in tests/test_stream.cpp
- same as ZMQ_ROUTER + ZMQ_ROUTER_RAW + ZMQ_ROUTER_MANDATORY
- includes b893ce set ZMQ_IDENTITY on outgoing connect
- deprecates ZMQ_ROUTER_RAW

Store identity for raw socket

Identity stored during connect procedure. Can be read using
zmq_getsockopt and used as the identity frame when sending messages.
This allows the implementation of a raw socket client.

Rename parse_properties to parse_metadata

Refactor how properties are parsed

Add test for CURVE mechanism

Test is skipped if no libsodium.
Added warning if libsodium not configured

Small code cleanup

Make ZAP optional for CURVE mechanism

Make ZAP optional for CURVE mechanism

Problem: allows CURVE security even if libzmq was built without libsodium