feat: persist ExpectedUtxo to disk (db)

closes #172, #137.

fixes loss-of-funds scenario.

changes:
 * add RustyWalletDatabase::expected_utxos (DbtVec)
 * remove UtxoNotificationPool
 * remove UtxoNotifier::PeerUnsigned
 * add WalletState methods:
    add_expected_utxo(), scan_for_expected_utxos()
 * disables timer call to prune_stale_expected_utxos() for now.
 + remove cli args:
    max_utxo_notification_size,
    max_unconfirmed_utxo_notification_count_per_peer
 * mod utxo_notification_pool --> expected_utxo
 * derive Hash for Timestamp

tests:
 * adapt existing tests to changes
 * move tests from utxo_notification_pool into wallet_state
 * adds regression tests for issue #172 that verify:
    1.  expected_utxo are persisted if db is written to disk.
    2.  expected_utxo are not persisted if db is not written to disk.

src/models/state/wallet/wallet_state.rs

@@ -305,13 +305,13 @@ impl WalletState {
     /// note: It is questionable if this method should ever be called
     ///       as presently implemented.
     ///
-    ///       issues:
-    ///         1. expiration does not consider if utxo has been
-    ///            claimed by wallet or not.
-    ///         2. expiration thresholds are based on time, not
-    ///            # of blocks.
-    ///         3. what if a deep re-org occurs after ExpectedUtxo
-    ///            have been expired?  possible loss of funds.
+    /// issues:
+    ///   1. expiration does not consider if utxo has been
+    ///      claimed by wallet or not.
+    ///   2. expiration thresholds are based on time, not
+    ///      # of blocks.
+    ///   3. what if a deep re-org occurs after ExpectedUtxo
+    ///      have been expired?  possible loss of funds.
     ///
     /// Fundamentally, any time we remove an ExpectedUtxo we risk a possible
     /// loss of funds in the future.