new encointerBalances.touch extrinsic to apply demurrage by non-owner

[brenzi]

If a community decides to change the demurrage rate, the new rate will only be applied lazily, which leads to inconsistencies in accounting.
In order to minimize inconsistencies we could introduce a lazy extrinsic that can be called by anyone which applies demurrage to an account.

community leadership could then apply demurrage to all accounts by a batch extrinsic, scheduled to execute shortly before enactment of the demurrage change. fees to be paid by sender