Shell: temporary legacy mempool

The shell library contains two mempool versions:

• The legacy mempool, implemented in all the files named legacy_..., is compatible with the Kathmandu protocol, and therefore it is needed for Mainnet.
• The new mempool relies on protocol functionalities that only exist since Lima.

When Lima is activated on Mainnet, every legacy_... file should be removed from lib_shell and lib_shell/test, so that only the new mempool remains.