Werk #10246: omd backup: Made it more robust against rrdcached communication issues

The "omd backup" could be terminated during backup by issues of the rrdcached. The backup communicates with the rrdcached to pause updates to the RRD files to ensure the files are consistent during backup. In case the rrdcached is not answering as expected or a connection is not possible the backup should continue with the backup instead of failing.

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