manager: use proper JSON values for socket communication

manager: use proper JSON values for socket communication

This commit adds a special JSON mode for control sockets.

The mode is activated by issuing a special `__json` command to the
socket, resulting in all Lua objects returned by all subsequent commands
to be serialized into JSONs, prepended by a 32-bit unsigned integer
byte-length value.

This JSON mode is now exclusively utilized by Manager, removing the need
to hackily strip single-quotes from the output and to read the output by
lines. Instead, it can always just read the 32-bit length value and
subsequently the whole JSON-formatted message, which is now
automatically deserialized into a Python object.