MB-6994: use only useful xdcr stats in xdcr tasks building
MB-6994: use only useful xdcr stats in xdcr tasks building

XDCR tasks do not need to have tons of stats. If somebody needs stats
there's API for stats. Tasks by design only expose 'progress' of
task. Thus we remove everything that we don't need for that.

Plus our code actually depended in a really hard way on those stats
attributes making hard backward-compat constrain for us.

Change-Id: I2f9dff24af8740453df62b6a6581446b4dd81a18
Reviewed-on: http://review.couchbase.org/21906
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Pavel Paulau <dd88eded64e90046a680e3a6c0828ceb8fe8a0e7@gmail.com>

pavel-blagodov/ns_server,nimishzynga/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,membase/ns_server,vzasade/ns_server,daverigby/ns_server,nimishzynga/ns_server,ceejatec/ns_server,mschoch/ns_server,daverigby/ns_server,nimishzynga/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,vzasade/ns_server,mschoch/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,membase/ns_server,vzasade/ns_server,mschoch/ns_server,membase/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,membase/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server