nodeDb.DeleteVersionsFrom can leave mutex locked indefinitely hence a deadlock

[odeke-em]

Noticed in an audit and while preparing a test for P #838 that this code exists

iavl/nodedb.go

Lines 493 to 499 in 3574b99

	ndb.mtx.Lock()
	for v, r := range ndb.versionReaders {
	if v >= fromVersion && r != 0 {
	return fmt.Errorf("unable to delete version %v with %v active readers", v, r)
	}
	}
	ndb.mtx.Unlock()

but in case the erroring condition matches, it'll be locked forever. Let's fix that by 2 ways

• A dedicated closure with an error return

err := func() error {
	ndb.mtx.Lock()
	defer ndb.mtx.Unlock()

	for v, r := range ndb.versionReaders {
		if v >= fromVersion && r != 0 {
			return fmt.Errorf("unable to delete version %v with %v active readers", v, r)
		}
	}
         return nil
}()

• Manually unlocking in the error path

	ndb.mtx.Lock()
	for v, r := range ndb.versionReaders {
		if v >= fromVersion && r != 0 {
                         ndb.mtx.Unlock()
			return fmt.Errorf("unable to delete version %v with %v active readers", v, r)
		}
	}
	ndb.mtx.Unlock()

/cc @elias-orijtech