fix xdcr stats names passed to UI

As described in the following doc,

https://developer.couchbase.com/documentation/server/current/rest-api/rest-xdcr-statistics.html

XDCR supports rest apis for retrieving xdcr runtime stats like the follows:

http://172.23.121.61:8091/pools/default/buckets/default/stats/replications%2Fe6e0658726d50980bf5651970c939c6e%2Fdefault%2Ftarget%2Fdocs_written

Things do not work for the following stats, though, since the stats name that xdcr passes to the UI are different from those described in the doc.

rate_replication in doc   -> rate_replicated  actual

docs_latency_wt in doc  -> wtavg_docs_latency

meta_latency_wt in doc -> wtavg_meta_latency

We need to change the stats names that xdcr passes to UI to match those in doc.