applier: unify state and ballot update triggers

Both triggers are used to wait for specific applier state (applier is
connected, remote node ballot is known, etc.) and are often used one
after another. Keeping these two trigger lists only adds complexity of
creating 2 triggers instead of 1 in some places. Let's unify the
triggers.

Now the triggers might be fired multiple times, so make all the existing
trigger functions reenterable.

In-scope-of #8509

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