vlog: Add a little more detail to ratelimit messages

vlog: Add a little more detail to ratelimit messages

When a message is suppressed by vlog ratelimiting, and then that message
occurs again much later, sometimes we get messages like this:

   Dropped 4 log messages in last 8695 seconds due to excessive rate

It seems pretty clear in this case that in fact we just didn't get that
kind of message for most of that 8695 seconds.  This commit improves the
message by adding a little more detail, e.g.:

   Dropped 4 log messages in last 8695 seconds (most recently, 6697 seconds
   ago) due to excessive rate.

Bug #2144.