Add message_t::routing_id() and set_routing_id()

[Tulon]

Setting a routing id is necessary when sending a message through a
ZMQ_SERVER socket. See [1] for more details.

This commit fixes #233.

[1] http://api.zeromq.org/4-2:zmq-socket#toc5

[coveralls]

Pull Request Test Coverage Report for Build 102

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+23.5%) to 78.182%

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Files with Coverage Reduction	New Missed Lines	%
zmq.hpp	2	64.1%

Totals
Change from base Build 95:	23.5%
Covered Lines:	129
Relevant Lines:	165

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💛 - Coveralls

[Tulon]

I am not quite sure why some CI builds fail. It looks like ZMQ_BUILD_DRAFT_API may be defined even though libzmq was built with draft APIs disabled.

[kurdybacha]

@Tulon thank you for PR. I would be able to take a look in the evening (GMT). Maybe someone else can help in the meantime. I would suggest you try to use ci_build.sh locally. Just run ./ci_build.sh without any arguments as this is one of the failing use cases (assuming you have Linux box). Maybe you get more info.

[sigiesec]

Hm, the poller stuff is additionally guarded by ZMQ_HAVE_POLLER, so maybe ZMQ_BUILD_DRAFT_API might really be always set. This might be an error on either the libzmq or our side.