feat: Meta HA fencing mechanism

[CAJan93]

Is your feature request related to a problem? Please describe.

A leader which looses its leader role should panic. This it to avoid a split-brain situation with 2 leaders.

When standby node becomes leader, it should wait for a lease expire interval ( at least keep lease successfully once) and then start to serve, which will ensure the leader have noticed the changes and gone. Thank you @yezizp2012 for the suggestion.

Also refactor MetaSrvEnv (see comment).