Merge remote-tracking branch 'origin/1.6' into 1.7

doc/sphinx/book/box/authentication.rst

doc/sphinx/book/box/box_space.rst

doc/sphinx/reference/digest.rst

doc/sphinx/reference/shard.rst

doc/sphinx/reference/socket.rst

doc/sphinx/reference/tarantool.rst

src/box/sophia_index.cc

src/lua/fio.c

src/lua/ipc.c

test/app/fio.result

test/app/fio.test.lua

test/app/ipc.result

test/app/ipc.test.lua

test/sophia/gh.result

test/sophia/gh.test.lua