Changes to comm_*_incoming()

Changes to comm_*_incoming()

Now, there is a single comm_incoming() function.  This does NOT use
select/poll.  It just calls the read handlers directly for HTTP
and ICP sockets, and relies on them to properly handle EWOULDBLOCK
and friends.

The rate at which comm_incoming gets called is configurable with
'incoming_min_rate' and 'incoming_max_rate'.   Note, these are
kind of "backwards" (being rates) and they indicate how many
other FD's to process before calling comm_incoming().  When the load
is none or low, we call comm_incoming at the 'min_rate' which defaults
to once every 16 FDs.  During high loads, 'max_rate' takes effect
and the default is every 2nd FD.

Also added a histogram to count how many connections/messages are
processed per call to comm_incoming.  This also percipitated some
StatHist.c changes.