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Introduce the traced_file class which wraps a file, adding CQL trace messages before and after every operation that returns a future.
Use this file to trace reads from SSTable data and index files.
Fixes#4908.
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* 'traced_file' of https://github.com/kbr-/scylla:
sstables: report sstable index file I/O in CQL tracing
sstables: report sstable data file I/O in CQL tracing
tracing: add traced_file class