Remove unavailable item from active sync requests

When got an `item_not_available_message` from a peer, the item was
removed from the `sync_items_requested_from_peer` list of the peer,
but it was left in the `_active_sync_requests` list.
When disconnecting from the peer, since the
`sync_items_requested_from_peer` list of the peer no longer contains
the item, the cleanup process does not remove it from the
`_active_sync_requests` list either.
As a result, the node will not request the item from another peer,
thus gets stuck.

libraries/net/node.cpp

@@ -2359,6 +2359,7 @@ namespace graphene { namespace net { namespace detail {
       auto sync_item_iter = originating_peer->sync_items_requested_from_peer.find(requested_item.item_hash);
       if (sync_item_iter != originating_peer->sync_items_requested_from_peer.end())
       {
+        _active_sync_requests.erase(*sync_item_iter);
         originating_peer->sync_items_requested_from_peer.erase(sync_item_iter);
 
         if (originating_peer->peer_needs_sync_items_from_us)