Merge branch '1.6' of git+ssh://github.com/tarantool/tarantool into 1.6

doc/sphinx/book/administration.rst

doc/sphinx/book/app_c_lua_tutorial.rst

doc/sphinx/book/app_d_plugins.rst

doc/sphinx/book/box/box_index.rst

doc/sphinx/book/box/box_introspection.rst

doc/sphinx/book/box/box_space.rst

doc/sphinx/book/replication/index.rst

doc/sphinx/reference/digest.rst

extra/dist/tarantoolctl

include/fiber.h

include/mutex.h

src/admin.cc

src/admin.rl

src/box/box_lua.cc

src/iproto.cc

src/lib/salad/rope.c

src/lib/salad/rope.h

src/log_io.cc

src/lua/fio.cc

src/lua/session.cc