fix: make pool shares transfers pausable

[lekhovitsky]

Fixes https://github.com/spearbit-audits/review-gearbox/issues/11

[StErMi]

The implementations now reverts the transfer of tokens when the Pool is paused by overriding the ERC20._transfer function.

A cleaner option would have been to override and implement the same logic in the _beforeTokenTransfer hook, but such hook would also have been called in other flows like ERC20._mint and ERC20._burn which are already covered directly in the PoolV3 implementation.

[lekhovitsky]

_transfer was chosen over _beforeTokenTransfer exactly to avoid duplicate checks (and make as little changes to the code as possible).