Add JSON RPC primitives for fetching the list of transactions that affect a given address

[mvines]

For the history that an API node maintains, it's not possible to fetch the list of transactions that affect a given address. This is a feature that is starting to be widely requested.

These two new JSON RPC methods could be path to enabling this feature:

1. getConfirmTransactionsForAddress. Input an address, return all the transaction signatures (paginated by slot?). For this we'll require a new rocksdb column to store the mapping of Address -> (Signature, Slot)

2. getConfirmedTransactionBySignature. Input a signature, return the transaction. This could be build on top of Blockstore's get_transaction_status()