XDCR: Use metakv sensitive APIs to set/add sensitive info.

metakv has now following two new APIs to set and add paths that store
sensitive information e.g. passwords.

1. SetSensitive(path string, value []byte, rev interface{})
2. AddSensitive(path string, value []byte)

Also, please do not display sensitive info in log files. I see goxdcr log displays passwords (cut-n-paste below).

AdminPort 2015-04-16T17:15:46.020-07:00 [INFO] doCreateRemoteClusterRequest$
AdminPort 2015-04-16T17:15:46.020-07:00 [INFO] Request params: justValidate=false, remoterClusterRef=

{remoteCluster/39cfb7cd7bac82eb471e644b6689c14a 39cfb7cd7bac82eb471e644b6689c14a backup 127.0.0.1:9002 Administrator asdasd false [] <nil>}

The new API and other changes in ns_server/metakv for storing sensitive info
were checked in under MB-14096.