Allow SHOW SERIES kill

[benbjohnson]

Previously, large SHOW SERIES queries would block until merging was complete before being interrupted. Interruption checks have been added to the iterator and the merge loop. I also tried adding interruption checks to the sort phase but it's difficult to break out of sort.Sort() upon interruption.

[stuartcarnie]

The change I made to Shards#CreateIterator will also need to wrap the SeriesPointIterator in an InterruptIterator

[stuartcarnie]

@benbjohnson would this also need to be wrapped?

influxdb/tsdb/shard.go

Lines 1254 to 1274 in 7b1c0e7

	func (a Shards) createSeriesIterator(ctx context.Context, opt query.IteratorOptions) (_ query.Iterator, err error) {
	var (
	idxs = make([]Index, 0, len(a))
	sfile *SeriesFile
	)
	for _, sh := range a {
	var idx Index
	if idx, err = sh.Index(); err == nil {
	idxs = append(idxs, idx)
	}
	if sfile == nil {
	sfile, _ = sh.seriesFile()
	}
	}
	if sfile == nil {
	return nil, errors.New("createSeriesIterator: no series file")
	}
	return NewSeriesPointIterator(IndexSet{Indexes: idxs, SeriesFile: sfile}, opt)
	}

[benbjohnson]

@stuartcarnie I don't think that section needs to be wrapped. I believe it gets wrapped by SELECT.