tsdb: reduce sleep time when reading WAL (#10859)
tsdb: reduce sleep time when reading WAL (#10859)

The code sleeps for a short time to allow goroutines to finish, however
it seems the duration can be reduced a lot, speeding up the reading
process.

I checked using some WAL data from production, and the queue is almost
always empty at the time we enter `waitForIdle()` so there is no danger
of spinning in the tight loop.

Signed-off-by: Bryan Boreham <de652539b24efdf404b95def4635bf3dbfd846aa@gmail.com>
prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus