contracts/PatriciaValidator: split _validatePatriciaProof into dedicated functions

[pinkiebell]

The goal of this change is that account proof and storage proofs can be separately verified.

Instead of verifying account and storage of the L1 bridge on L2 for each message, verify the account proof once and save the last known storage root of the L1 bridge account (on L2 bridge).

For each message delivery(on L2), only submit a storage proof and validate against that.