CBD-792: Change prefix of XDCR keys

- Changing prefix for keys used to
determine xdcr lag
- By doing so, we will be avoiding dumping
timing traces in logs for sentinel keys (those
starting with XDCR_TIMING_)
- New prefix: xdcr_track_

Change-Id: Id2ca4112e8a7aad869edff5dd962cfb23795df42
Reviewed-on: http://review.couchbase.org/25026
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: abhinav dangeti <abhinav.dangeti@gmail.com>