Add caching support for transaction and event proofs

[Mirko-von-Leipzig]

This should be done last.

Add caching to the transaction and event proofs. We don't store the merkle trie for these during sync - we only calculate them once off to verify the commitment values match. These RPC methods will therefore be expensive since they re-calculate the commitments for each call.

Adding caching will greatly improve matters here. This cache should also include values from the sync as these are likely to be popular.

We probably want request coalescing since it is likely that proofs for the same block will occur multiple times.