Move fts diagnostics from couchbase.log to fts_diag.json.
Move fts diagnostics from couchbase.log to fts_diag.json.

Because the output of /api/diag can be massive which makes emacs'
incremental search struggle. Other than that, it makes it easier to fts
folks to query the information they need from the response.

Change-Id: I8435633a77f58ed3ffe7b1a6db896d246222119d
Reviewed-on: http://review.couchbase.org/63809
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server