fixes #4 Statistics support

This introduces new public APIs for obtaining statistics,
and adds some generic stats for dialers, listeners, pipes, and
sockets.  Also added are stats for inproc and pairv1 protocol.

The other protocols and transports will have stats added
incrementally as time goes on.

A simple test program, and man pages are provided for this.
Start by looking at nng_stat(5).

Statistics does have some impact, and they can be disabled by
using the advanced NNG_ENABLE_STATS (setting it to OFF, it's
ON by default) if you need to build a minimized configuration.

1 files changed, 60 insertions, 0 deletions

diff --git a/docs/man/nng_stat_timestamp.3.adoc b/docs/man/nng_stat_timestamp.3.adoc
new file mode 100644
index 00000000..f23d8720
--- /dev/null
+++ b/docs/man/nng_stat_timestamp.3.adoc
@@ -0,0 +1,60 @@
+= nng_version(3)
+//
+// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
+// Copyright 2018 Capitar IT Group BV <info@capitar.com>
+//
+// This document is supplied under the terms of the MIT License, a
+// copy of which should be located in the distribution where this
+// file was obtained (LICENSE.txt).  A copy of the license may also be
+// found online at https://opensource.org/licenses/MIT.
+//
+
+== NAME
+
+nng_stat_timestamp - get statistic timestamp
+
+== SYNOPSIS
+
+[source, c]
+----
+#include <nng/nng.h>
+
+typedef struct nng_stat nng_stat;
+
+uint64_t nng_stat_timestamp(nng_stat *stat);
+----
+
+== DESCRIPTION
+
+The `nng_stat_timestamp()` function returns the a timestamp, which is measured
+as a number of milliseconds since some arbitrary value in the past.
+
+Even within the same snapshot, statistics might have different timestamp
+values, as the timestamp represents the time when that particular statistic
+was captured.
+
+TIP: These values are intended to facilitate calculation of rates, by
+comparing values from one snapshot with a subsequent one.
+
+TIP: The values used here have the same offset as the
+`<<nng_clock.3supp#,nng_clock()>>` supplementation function.
+This can be useful when converting these values to local clock time.
+
+== RETURN VALUES
+
+The timestamp when _stat_ was captured, measured as a number of
+milliseconds since some time in the past.
+
+== ERRORS
+
+None.
+
+== SEE ALSO
+
+[.text-left]
+<<libnng.3#,libnng(3)>>,
+<<nng_stats_get.3#,nng_stats_get(3)>>,
+<<nng_stat_value.3#,nng_stat_value(3)>>,
+<<nng_clock.3supp#,nng_clock(3supp)>>,
+<<nng_stat.5#,nng_stat(5)>>,
+<<nng.7#,nng(7)>>