db: audit ctx.Background() uses, adopt cancellation on Close()

[RaduBerinde]

loadNewFileStats might take a long time if stats collection is running behind (e.g. sstables are backed by object storage). We only check d.closed in the beginning, then try to process all updated files:

pebble/table_stats.go

Line 165 in d8d0e1e

func (d *DB) loadNewFileStats(

Jira issue: PEBBLE-1346

[RaduBerinde]

We should set up the context properly too (and not report errors due to cancellation).

[RaduBerinde]

#5774 adds a check in-between each file. The issue tracks making this better by using a cancelable context.

[dt]

I've retitled this since the specific function is checking closed now.

I think ~every context.Background() is dubious. We should probably make a new context.WithCancel when we open DB, store it in db and then every Background() should be switched d.backgroundCtx , with db.Close() learning to cancel it.