Increase the call's MIN_RETAINED_GAS

[chfast]

The currently proposed value of MIN_RETAINED_GAS is 5000. This is exactly the cost of updating a "fresh" storage slot (so it cannot be done) but one can update a storage slot that is worm or has been updated/added before the call.

I think we should increase MIN_RETAINED_GAS to the value always allowing updating a storage slot. Also consider adding a new storage value (20000).

[axic]

How about also defining the retained gas not by a fixed number, but rather as i.e. "maximum cost of SSTORE * 1.5x" ?

[chfast]

(the legacy CALL has min retained gas 0)

[chfast]

The script for the analysis: #97.
Please review the math there.