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

closes Neptune-Crypto#189.

input selection now ignores spent inputs from unconfirmed tx in the
mempool.

Also fixes an input selection bug where the balance check considers
available funds (not timelocked) but the input utxo selection does not.

Adds a test to verify that same input can no longer be spent twice.