MINOR: sink: implement "show events" to show supported sinks and dump the rings

The new "show events" CLI keyword lists supported event sinks. When
passed a buffer-type sink it completely dumps it.

no drops at all during attachment even at 8 millon evts/s.
still missing the attachment limit though.
diff --git a/doc/management.txt b/doc/management.txt
index b54b30c..8b3f6f6 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -1964,6 +1964,11 @@
     is the slash ('/') in header name "header/bizarre", which is not a valid
     HTTP character for a header name.
 
+show events [<sink>]
+  With no option, this lists all known event sinks and their types. With an
+  option, it will dump all available events in the designated sink if it is of
+  type buffer.
+
 show fd [<fd>]
   Dump the list of either all open file descriptors or just the one number <fd>
   if specified. This is only aimed at developers who need to observe internal