MB-3186 use server time zone for timestamps of log entries

backward compatibility story :

- after the ns_log file is unzipped all entries will
  be upgraded to the new format
- if the pre 3.0 node will send do_log or sync to 3.0 node
  the log entries will be upgraded and time zone
  of the receiving node will be used
- 3.0 node will strip server_time from the log
  entries when it sends do_log or sync to pre 3.0 nodes

Change-Id: I070a22eb238229357f83b80cc382637689661998
Reviewed-on: http://review.couchbase.org/29267
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>