Prevent slave data delete when unable to connect to master

[nojima]

This PR extracts bug-fix commits from #90.

These commits fixes the following issues:

1. When a slave node disconnected from master longer than 5 seconds, it clears all data it stores. That is, when a leader election takes more than 5 seconds, all data are lost.
2. When an error occurred in a name resolution in tcp_connect, yrmcds crushes without retry. This is an issue because we use dynamically created/updated Service resources to connect to the master node on Kubernetes cluster.
3. When slave cannot to promote to the master, yrmcds outputs too many logs, which are not useful.

[nojima]

CI failed with the following message. We need to remove 20.04 from CI.

This is a scheduled Ubuntu 20.04 retirement. Ubuntu 20.04 LTS runner will be removed on 2025-04-15.

[ymmt2005]

LGTM