Merge pull request #997 from soundart/master

tweetnacl integration for curve
Don't add empty metadata to received messages
Merge pull request #998 from hurtonm/master

Don't add empty metadata to received messages
Include ZMTP properties in message metadata

Metadata are built in stream_engine now.
This makes it easy to extend metadata with user-defined properties.
Merge pull request #999 from hurtonm/master

Include ZMTP properties in message metadata
Add code to investigate data race

Provides control of race windows and logging for some of the interesting transitions.
Enlarge the race window for some test cases

These changes cause an assertion in lb_sendpipe() when executing the test_security_plain testcase.
Merge pull request #1000 from daveab/feature/divide_by_zero_investigation

Feature/divide by zero investigation
Revert "Enlarge the race window for some test cases"

This reverts commit 2a84d2597469d272794a1fb5fb43c6260f335195.

Broke regression test on test_hwm.cpp.
Revert "Add code to investigate data race"

This reverts commit 5e0facda17b986943f0fe40a2dc6c98611655c35.
Merge pull request #1001 from hintjens/master

These commits broke regression tests, so I'm reverting them.
Added test_metadata

- also prioritize ZAP metadata over ZMTP metadata in case application
  uses same names.
Fixed compile error in test_security_null
Problem: GSSAPI server doesn't parse ZAP user id

Solution: add this to ZAP reply processing.
Fixed man page example
ZAP vs. ZMTP metadata priority fixed
Merge pull request #1003 from hintjens/master

Added test_metadata
Initialise _has_src_addr variable, fixes hang on Windows.
Merge pull request #1005 from ricnewton/master

Initialise _has_src_addr variable, fixes hang on Windows.
Remove i_properties interface

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