wallet subscribes to mempool via tokio broadcast to track owned utxos.

Changes:

 * add spent_utxos, unspent_utxos lists to WalletState struct
 * add WalletState::handle_mempool_event()
 * add test confirmed_and_unconfirmed_balance()
 * add Mempool::event_channel (tokio broadcast channel)
 * Mempool faillible mutation methods return Result()
 * Mempool mutable methods broadcast MempoolEvent
 * add tests::shared::mine_block_to_wallet()
 * lib.rs: spawn wallet task for listening to mempool events
   and dispatch to WalletState for handling.
 * add locks::tokio::AtomicRw::try_lock_guard_mut()

removes the mempool broadcast channel and wallet listener task.

Instead all mempool mutations go through GlobalState methods
which inform wallet of the changes.  This makes changes atomic
over mempool+wallet so they are always in sync.

Changes:
 * remove Mempool::event_channel
 * Mempool &mut methods only callable by super
 * Mempool &mut methods return MempoolEvent(s)
 * add MempoolEvent::UpdateTxMutatorSet.  (unused)
 * add GlobalState methods: mempool_clear, mempool_insert,
    mempool_prune_stale_transactions
 * remove spawn_wallet_task from lib.rs
 * add/improve doc-comments