Integrate account fetching into SmartContracts

[mitschabaude]

• Use the existing fetch methods magically within createTransaction and similar places to fetch account state from an API
• Add a cache to store existing fetch results in
• Refactor createTransaction, such that it either uses account state from the cache, or marks it as "to be fetched" (not fetch it directly!). For proof transactions, this enables us to first run it "test mode", then asynchronously fetch the missing data, then run it again (with all the data available) inside the synchronous prover. For non-proof transactions, there can be a simple async wrapper that does those three steps in sequence.
• Expose a method to explicitly populate the cache with arbitrary data, for testing scenarios / optional fine-grained control over where the data comes from (e.g. from disk instead of from an API).

Small TODO which should be enabled by this:

• Add nonce precondition to all signed parties - currently only done in createSigned