index-speedup: code review.

include/errcode.h

include/mhash.h

src/box/box.m

src/box/box_lua.m

src/box/index.h

src/box/index.m

src/box/request.m

src/box/space.h

src/box/space.m

src/box/tree.m

src/box/txn.h

src/box/txn.m

test/big/hash.result

test/big/lua.result

test/big/tree_pk.result

test/box/errinj.result

test/box/fiber.result

test/box/lua.result

third_party/sptree.h