Renumber box request ids from 0 to 6 in 1.6.

extra/empty.snap

src/bootstrap.snap

src/box/lua/box.lua

src/box/lua/box_net.lua

src/box/request.cc

src/box/request.h

src/iproto.cc

src/iproto_port.h

test/box/admin.result

test/box/dup_key1.xlog

test/box/dup_key2.xlog

test/box/fiber.result

test/box/fiber.test.lua

test/box/iproto.test.py

test/box/lua.result

test/box/lua.test.py

test/box/lua_misc.result

test/box/net.box.result

test/box/socket.result

test/box/socket.test.py