Emit event when changing total locked value in pallet-balances (#12287)

* Emit Locked/Unlocked events

* Implement lock event tests

* Adhere to style guide

* Use saturating math

Co-authored-by: Kian Paimani <[email protected]>

* Fix typo

* Emit event on change locks and add tests

* Adjust event docstring

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Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: parity-processbot <>