Werk #7541: rrdcached: added commands to suspend writing to rrd files

using these commands, one can backup rrd files in a consistent state. There are four new commands in total, 2 for suspend, 2 for resume, one each applying to individual files (SUSPEND/RESUME) and one each for all files at once (SUSPENDALL/RESUMEALL). Please note that these mechanisms are independent from each other so you can't use RESUMEALL to release files suspended with SUSPEND

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