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, 86 insertions, 0 deletions

diff --git a/docs/man/nng_stats_get.3.adoc b/docs/man/nng_stats_get.3.adoc
new file mode 100644
index 00000000..caf91dc3
--- /dev/null
+++ b/docs/man/nng_stats_get.3.adoc
@@ -0,0 +1,86 @@
+= nng_alloc(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_stats_get - get statistics snapshot
+
+== SYNOPSIS
+
+[source, c]
+----
+#include <nng/nng.h>
+
+typedef struct nng_stat nng_stat;
+
+int nng_stats_get(nng_stat *statsp)
+----
+
+== DESCRIPTION
+
+The `nng_stat_get()` function attempts to obtain a snapshot of all the
+various diagnostic statistics that are present in the system.
+
+NOTE: The process of collecting statistics is designed to have minimal
+impact on the system, but there is still some impact.
+
+The statistics are organized as a tree, rooted with a parent
+statistic of type `NNG_STAT_SCOPE` that carries no value, and which
+has an empty name.
+This parent statistic is returned through the _statsp_ pointer.
+
+The `<<nng_stat_child.3#,nng_stat_child()>>` and
+`<<nng_stat_next.3#,nng_stat_next()>>` function can be used to
+iterate over the the tree.
+
+When no longer needed, the statistics can be freed with the
+`<<nng_stats_free.3#,nng_stats_free()>>` function, but that
+function must be called only with the root statistic that is returned
+through the _statsp_ pointer.
+
+NOTE: The values of individual statistics are guaranteed to be atomic,
+but due the way statistics are collected there can be discrepancies between them at certain times.
+For example, statistics counting bytes and messages received may not
+reflect the same number of messages, depending on when the snapshot is taken.
+This potential inconsistency arises as a result of optimizations to minimize
+the impact of statistics on actual operations.
+
+NOTE: The names, values, and semantics of statistics provided may change
+from release to release.
+These are provided for informational and debugging use only, and applications
+should not rely on the presence, names, or meanings of any individual statistics.
+
+== RETURN VALUES
+
+This function returns a pointer to the allocated memory on success,
+and `NULL` otherwise.
+
+== ERRORS
+
+[horizontal]
+`NNG_ENOMEM`:: Insufficient free memory to collect statistics.
+`NNG_ENOTSUP`:: Statistics are not supported (compile time option).
+
+== SEE ALSO
+
+[.text-left]
+<<nng_stats_free.3#,nng_stats_free(3)>>,
+<<nng_stat_child.3#,nng_stat_child(3)>>,
+<<nng_stat_desc.3#,nng_stat_desc(3)>>,
+<<nng_stat_name.3#,nng_stat_name(3)>>,
+<<nng_stat_next.3#,nng_stat_next(3)>>,
+<<nng_stat_string.3#,nng_stat_string(3)>>,
+<<nng_stat_type.3#,nng_stat_type(3)>>,
+<<nng_stat_timestamp.3#,nng_stat_timestamp(3)>>,
+<<nng_stat_unit.3#,nng_stat_unit(3)>>,
+<<nng_stat_value.3#,nng_stat_value(3)>>,
+<<nng_stat.5#,nng_stat(5)>>,
+<<nng.7#,nng(7)>>