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".

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These classes are intended for use server-side.

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e.g. in a django view.py :

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    from pyjs.jsonrpc.django import JSONService, jsonremote

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    jsonservice = JSONRPCService()

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    @jsonremote(jsonservice)

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    def test(request, echo_param):

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         return "echoing the param back: %s" % echo_param

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