Add retain basic implementation on GNU/Linux

Retain is saved in file and backup of this file is done every time.
File isn't saved on every PLC cycle to avoid high I/O load.
By default, retain is saved every second and on PLC stop/shutdown.
It's possible to override retain flush period by using CFLAGS option
in project's settings. For example, for 0.5 seconds period
use "-DFILE_RETAIN_SAVE_PERIOD_S=0.5".

# pylint: disable=undefined-variable

from __future__ import absolute_import
from svgui.pyjs.jsonrpc.jsonrpc import JSONRPCServiceBase


class JSONRPCService(JSONRPCServiceBase):

    def serve(self):
        return self.process(request.body.read())

    def __call__(self, func):
        self.methods[func.__name__] = func
        return func