Make vault client attempt reconnect to overlay network if periodic health check fails

[ebma]

Context

Currently, the vault client logs periodic health check messages if it continues to receive SCP messages from the overlay node. If the health check fails (ie no SCP message is received within some timeframe), the vault doesn't act on this. This causes other issues as for example the check for the recoverable slot here might always return true even if the slot is long past, just because the last_slot_index doesn't update anymore.

TODO

A simple solution would be to send a shutdown signal and have the vault client restart.