Merge branch 'btree-v2'

src/box/tree_index.cc

src/box/tree_index.h

src/lib/salad/bps_tree.h

src/lib/small/pt_alloc.c

src/lib/small/pt_alloc.h

test/big/tree_pk.result

test/big/tree_pk.test.lua

test/box/errinj_index.result

test/box/errinj_index.test.lua

test/lib/tarantool-python

test/unit/CMakeLists.txt

test/unit/bps_tree.cc

test/unit/bps_tree.result

test/unit/bps_tree_itr.cc

test/unit/bps_tree_itr.result