sstable: use fixed error in Close

[dt]

Previously sstable.Reader.Close() would initialize a new error each time it was called.

In workloads that open and close many readers, this could actually have non-trivial
expense.

Instead, this change adds a pre-initialized error value and just assigns it in Close().

[cockroach-teamcity]

This change is 

[itsbilal]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @dt and @itsbilal)

---

sstable/reader.go, line 1638 at r1 (raw file):

}

var errReaderClosed = errors.New("pebble/table: reader is closed")

Nit: I'd move this to being right below errCorruptIndexEntry.

[dt]

TFTR!