contracts: Cap the surcharge reward by the amount of rent that way payed by a contract (#7870)

* Add rent_payed field to the contract info

* Don't pay out more as reward as was spent in rent

* Make successful evictions free

* Add tests to check that surcharge reward is capped by rent payed

* review: Fixed docs

substrate/frame/contracts/src/storage.rs

@@ -27,7 +27,7 @@ use codec::{Encode, Decode};
 use sp_std::prelude::*;
 use sp_std::marker::PhantomData;
 use sp_io::hashing::blake2_256;
-use sp_runtime::traits::{Bounded, Saturating};
+use sp_runtime::traits::{Bounded, Saturating, Zero};
 use sp_core::crypto::UncheckedFrom;
 use frame_support::{
 	dispatch::DispatchResult,
@@ -181,6 +181,7 @@ where
 						// charge rent for it during instantation.
 						<frame_system::Module<T>>::block_number().saturating_sub(1u32.into()),
 					rent_allowance: <BalanceOf<T>>::max_value(),
+					rent_payed: <BalanceOf<T>>::zero(),
 					pair_count: 0,
 					last_write: None,
 				}