Public mempool API to query the size limit

[deepfire]

End users would benefit from obtaining information regarding the limit of the mempool, such that, for example, the MempoolSize (and, hence MempoolSnapshot) could be extended to contain not only the its current byte/Tx occupancy, but also the byte limit:

-- | The size of a mempool.
data MempoolSize = MempoolSize
  { msNumTxs   :: !Word32
    -- ^ The number of transactions in the mempool.
  , msNumBytes :: !Word32
    -- ^ The summed byte size of all the transactions in the mempool.
  , msMaxBytes :: !Word32
    -- ^ The upper limit on the byte size of the mempool.
  } deriving (Eq, Show)

[deepfire]

implementation: #1509