Introduce stats log API, convert existing output

Convert regular 'stats.log' output to this new API.

In addition to the current stats value, also give the last value. This
makes it easy to display the difference.
stats: introduce global config

As the stats api calls the loggers at a global interval, the global
interval should be configured globally.

 # global stats configuration
 stats:
   enabled: yes
   # The interval field (in seconds) controls at what interval
   # the loggers are invoked.
   interval: 8

If this config isn't found, the old config will be supported.
stats: initialize after outputs

Initialize stats after outputs so that we can check if we need to
initialize the stats api at all.
stats: disable stats if no loggers are enabled
output streaming: cleanup at runmode destruction
stats: expose stats to Lua output

Register with type 'stats':

    function init (args)
        local needs = {}
        needs["type"] = "stats"
        return needs
    end

The stats are passed as an array of tables:

    { 1, { name=<name>, tmname=<tm_name>, value=<value>, pvalue=<pvalue>}}
    { 2, { name=<name>, tmname=<tm_name>, value=<value>, pvalue=<pvalue>}}
    etc

Name is the counter name (e.g. decoder.invalid), tm_name is the thread name
(e.g. AFPacketeth05), value is current value, and pvalue is the value of the
last time the script was invoked.
MemBuffer: add expansion call

For some of the buffer users it's hard to predict how big the data
will be. In the stats.log case this depends on chosen runmode and
number of threads.

To deal with this case a 'MemBufferExpand' call is added. This realloc's
the buffer.
log-stats: expand membuffer if necessary

Many threads could lead to a membuffer size requirement bigger than
64k. So use the expansion call to grow the buffer as needed.