Safe asset disabling

[mootz12]

Disabling assets is currently a difficult task for pool managers. If an asset has a liability factor, disabling it completely can be almost impossible (setting the liability factor to 0 results in positions that cannot be calculated. If these already exist these users will not be able to have their health factor calculated), and they rather need to be set with an extremely high liability factor and 0 collateral factor.

Technical Notes

This is likely best implemented with an enabled flag on the ReserveConfig, where it being disabled only allows the destruction of positions (withdraw / repay).