Currently stop returns ready future immediately. This is not a problem
since calculation loop holds a shared pointer to the local service, so
it will not be destroyed until calculation completes and global database
object db, that also used by the calculation, is never destroyed. But the
later is just a workaround for a shutdown sequence that cannot handle
it and will be changed one day. Make cache hitrate calculation service
ready for it.
Message-Id: <20190422113538.GR21208@scylladb.com>
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