Facebook sync (May 2019) + relicense (#838)

Changelog:

- changed license: BSD+Patents -> MIT
- propagates exceptions raised in sub-indexes of IndexShards and IndexReplicas
- support for searching several inverted lists in parallel (parallel_mode != 0)
- better support for PQ codes where nbit != 8 or 16
- IVFSpectralHash implementation: spectral hash codes inside an IVF
- 6-bit per component scalar quantizer (4 and 8 bit were already supported)
- combinations of inverted lists: HStackInvertedLists and VStackInvertedLists
- configurable number of threads for OnDiskInvertedLists prefetching (including 0=no prefetch)
- more test and demo code compatible with Python 3 (print with parentheses)
- refactored benchmark code: data loading is now in a single file

MetaIndexes.cpp

@@ -1,8 +1,7 @@
 /**
- * Copyright (c) 2015-present, Facebook, Inc.
- * All rights reserved.
+ * Copyright (c) Facebook, Inc. and its affiliates.
  *
- * This source code is licensed under the BSD+Patents license found in the
+ * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  */