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 https://github.com/cosmos/iavl/blob/3574b99d1cb0f06591c2bd02cba47bef7ebc9312/nodedb.go#L493-L499

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