added more diagnostics to xdcr xmem flushing
added more diagnostics to xdcr xmem flushing

So that we can see exactly what was actually sent to remote end

Change-Id: I7bfaec8843c0071d31e1c7db372038605aabc5d6
Reviewed-on: http://review.couchbase.org/35715
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

daverigby/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,nimishzynga/ns_server,daverigby/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,nimishzynga/ns_server,daverigby/ns_server