Implement store-gateway block closing

[simonswine]

When serving queries from bucket store-gateways open blocks, but they are never closed:

pyroscope/pkg/storegateway/query.go

Line 199 in 0f75877

if err := querier.Open(ctx); err != nil {

We should implement reference counting and also define a block idle time.:

pyroscope/pkg/phlaredb/block_querier.go

Lines 2083 to 2094 in 0f75877

	func (q *singleBlockQuerier) Open(ctx context.Context) error {
	q.openLock.Lock()
	defer q.openLock.Unlock()
	if !q.opened {
	if err := q.openFiles(ctx); err != nil {
	return err
	}
	}
	q.metrics.blockOpened.Inc()
	q.opened = true
	return nil
	}

Reported via the community slack in thread: