proxy: Update timestamp partition on slave changes

Each time slave partition changes (it is added or updated) the master
partition timestamp has to be recalculated.

/reviewed-on !176
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Status Job ID Name Coverage
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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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failed SlapOS.Eggs.UnitTest-Master.Python3

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