closes#7.
adds a blockchain watchdog that checks every hour if the tip height has
advanced or not. Sends alert email if height is less or equal to
height at the last check and enters a warning mode. In warning mode
it waits until the height is greater than previous, and then sends a
recovery alert and switches to normal mode.
closes#1
Implements a background watchdog task that:
1. calls neptune-core /network rpc every N seconds (default: 10)
2. emits log message on any state change
3. sends detailed email alert to admin on any state change
Changes:
* AppState now has internal Arc<RwLock<AppStateInner>> to permit
watchdog task to mutate the rpc_client field.
* adjust application to AppState changes (use locks)
* not_found functions no longer accept state arg
* move state initialization into AppState::init()
* add optional alert parameters: admin-email, smtp-*
* add neptune_rpc module with watchdog task
* add alert_email module
* simplify main()
* log warnings if alert parameters not set
* add chrono dep
* add lettre dep
