Review the use of `parking_lot` in mempool

[TheQuantumPhysicist]

(Created on Aug 20, 2023 by @iljakuklic)

Mempool currently uses parking_lot::RwLock for rolling fee rate. It seems as a workaround to provide interior mutability access to the rolling fee rate because it is computed lazily. RwLock does not seem the best match for the job. Either find a more appropriate alternative (e.g. one that implements the notion of lazy computation) or find a way to avoid interior mutability altogether.