Show mempool stats #2

Show basic mempool stats, like number of items and size in bytes.

Other more advanced details could track this data over time, or view details inside mempool tx's

For now, this ticket will just be for rendering this bits of data on a dedicated /mempool/ page:

Size of mempool in bytes
Size of mempool in transactions
Amount of total fees in tx's in the mempool
List of txid's in the mempool

The list of txid's in mempool will not have a link to a tx view page, because we won't render the data about tx's in the mempool yet. That is because we don't have a way of updating the data to say if a txid was orhpaned by a block re-org.

The above info is available from the getrawmempool and getmempoolinfo RPCs.