Add support for xdcr latency metrics

[evanlucas]

{
    "specificStatsURL": "/pools/default/buckets/biscuits/stats/replications%2F585e252deb2935538d3652f0da3c5acc%2Fbiscuits%2Fbiscuits%2Fwtavg_meta_latency",
    "title": "ms meta batch latency",
    "name": "replications/585e252deb2935538d3652f0da3c5acc/biscuits/biscuits/wtavg_meta_latency",
    "desc": "Weighted average latency in ms of sending getMeta and waiting for conflict solution result from remote cluster (measured from per-replication stat wtavg_meta_latency)"
},
{
    "specificStatsURL": "/pools/default/buckets/biscuits/stats/replications%2F585e252deb2935538d3652f0da3c5acc%2Fbiscuits%2Fbiscuits%2Fwtavg_docs_latency",
    "title": "ms doc batch latency",
    "name": "replications/585e252deb2935538d3652f0da3c5acc/biscuits/biscuits/wtavg_docs_latency",
    "desc": "Weighted average latency in ms of sending replicated mutations to remote cluster (measured from per-replication stat wtavg_docs_latency)"
}

The bucket statsDirectory should include them under the Outbound XDCR blocks