Bring `mempoolinfo` up to date

[roylee17]

On lbcd

{
  "size": 76,
  "bytes": 54793
}

On lbrycrd:

{
  "size": 157,
  "bytes": 127988,
  "usage": 267040,
  "maxmempool": 300000000,
  "mempoolminfee": 0.00001000,
  "minrelaytxfee": 0.00001000
}

Latest spec:

{                            (json object)
  "loaded" : true|false,     (boolean) True if the mempool is fully loaded
  "size" : n,                (numeric) Current tx count
  "bytes" : n,               (numeric) Sum of all virtual transaction sizes as defined in BIP 141. Differs from actual serialized size because witness data is discounted
  "usage" : n,               (numeric) Total memory usage for the mempool
  "total_fee" : n,           (numeric) Total fees for the mempool in BTC, ignoring modified fees through prioritizetransaction
  "maxmempool" : n,          (numeric) Maximum memory usage for the mempool
  "mempoolminfee" : n,       (numeric) Minimum fee rate in BTC/kvB for tx to be accepted. Is the maximum of minrelaytxfee and minimum mempool fee
  "minrelaytxfee" : n,       (numeric) Current minimum relay fee for transactions
  "unbroadcastcount" : n     (numeric) Current number of transactions that haven't passed initial broadcast yet
}