Switched all sysdb.io links to https://
timeseries::rrdtool: Added support for RRDCacheD. After enabling RRDCacheD support, the plugin supports an additional time-series fetcher callback which may be used to flush outstanding updates to an RRD file before fetching data from it.
doc/: Improved documentation around the rrdtool time-series fetcher.
Added documentation for the new timeseries::rrdtool plugin.