MB-7104: account failed checkpoints stats correctly
MB-7104: account failed checkpoints stats correctly

The current code incorrectly computes the failed XDCR checkpoints
and in some cases, the stat on UI is always 0 and does not show
the real number of failed checkpoints. This change-set will fix
this issue.

Change-Id: Iad48dbeb3238675263cd816e9299c0cb7e369a50
Reviewed-on: http://review.couchbase.org/22297
Reviewed-by: Damien Katz <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@couchbase.com>
Reviewed-by: Junyi Xie <cec9b1c9f34a8d92cba938f3f3364d5120580782@gmail.com>
Tested-by: Junyi Xie <cec9b1c9f34a8d92cba938f3f3364d5120580782@gmail.com>

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