BUG/MINOR: stats: fix show stat json buffer limitation

BUG/MINOR: stats: fix show stat json buffer limitation

json output type is a lot more verbose than other output types.
Because of this and the increasing number of metrics implemented within
haproxy, we are starting to reach max bufsize limit (defaults to 16k)
when dumping stats to json since 2.6-dev1.
This results in stats output being truncated with
    "[{"errorStr":"output buffer too short"}]"

This was reported by Gabriel in #1964.

Thanks to "MINOR: stats: introduce stats field ctx", we can now make
multipart (using multiple buffers) dumping, in case a single buffer is not big
enough to hold the complete stat line.
For now, only stats_dump_fields_json() makes use of it as it is by
far the most verbose stats output type.
(csv, typed and html outputs should be good for a while and may use this
capability if the need arises in some distant future)

--

It could be backported to 2.6 and 2.7.
This commit depends on:
  - MINOR: stats: provide ctx for dumping functions
  - MINOR: stats: introduce stats field ctx