Fix #328: Move `session` to `box`

src/CMakeLists.txt

src/box/CMakeLists.txt

src/box/box.cc

src/box/lua/call.cc

src/box/lua/schema.lua

src/box/lua/session.cc

src/box/lua/session.h

src/box/lua/session.lua

src/box/session.cc

src/box/session.h

src/iproto.cc

src/lua/console.lua

src/lua/init.cc

src/tarantool.cc

test/app/session.test.lua

test/box/.box.lua.kate-swp

test/box/access.result

test/box/access.test.lua

test/box/auth_access.result

test/box/auth_access.test.lua