fix(tsi): retaining fileset should not copy pointer

[foobar]

Current implementation of retainFileSet is not safe because it returns p.fileSet directly. It is supposed to be protected by mutex:

func (p *Partition) retainFileSet() *FileSet {
	fs := p.fileSet
	fs.Retain()
	return fs
}

The bug may lead to deadlock if the fileset is modified while there is compaction.

[foobar]

The CI will success when https://github.com/influxdata/influxdb/pull/18961 is merged

[foobar]

@dgnorton I suppose this fix needs to be ported back to 1.7/1.8

[davidby-influx]

@foobar - can you make a unit test for this showing the problem?

[foobar]

rereview the code, FileSet.files is actually immutable so the clone is not needed.