Cache transactions retrieved after a pool request

When publishing a new tails file, the server needs to read the current state of the targeted ledger in order to check the state of the revocation registry.

Every time a new file is pushed, the server needs to "start from scratch" and re-read all of the transactions from inception on the specified ledger (via the genesis file specified during upload + new entries on that ledger).

To speed-up this process, it would be nice to have the last know state for a specific ledger cached so that only transactions newer than the last update - if any - will need to be fetched and refreshed.