- add const modifier to address parameter of event generating functions

Applications that use ZMQ_IDENTITY can be trapped by the artificial
restriction on not using a binary zero as first byte. It's specially
nasty on random generated identities, e.g. UUIDs, as the chance of a
binary zero is low, so it will pass 255 out of 256 times.

Solution: remove the restriction.

NULL mechanism sends ERROR command rather than READY command when ZAP
handler rejects the ZAP request (status code != "200"). The body of
ERROR command contains the status code as returned by ZAP handler.

Using 'ipc://@abstract-socket' on non-Linux platforms yields inconsistent
behaviour.  Abstract sockets don't exist, so the literal file is created.
The test previously failed, but for a different reason: this is not the
directory you are looking for.  Now, zmq_bind() will fail for the right
reason: the socket can't be created.  Put the XFAIL back.

Tested on FreeBSD 10.

We use metadata_t directly. No need for generic interface now.

Solution: add this to ZAP reply processing.

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.

This reverts commit 5e0facda.

Provides control of race windows and logging for some of the interesting transitions.

Metadata are built in stream_engine now.
This makes it easy to extend metadata with user-defined properties.