When sync mode is exited, request a block notification from peers

When syncing mode is exited, the client might still be missing some blocks as syncing mode is only used when the client is ~50 to ~100 blocks behind a peer, where the precise number is determined by max_number_of_blocks_before_syncing value which can be set from the CLI and defaults to 100.

Therefore, a client should send a BlockNotificationRequest to some of its peers after syncing mode is abandoned, to catch up with the few remaining blocks.

The relevant place that mutates the syncing value is in the handle_peer_thread_message method in main_loop.rs