Simplify persistence design for temporarily offline status

[Vlatombe]

There were complex code to synchronize transient state of Computer from Node and vice versa.
But since any temporarily offline cause is being set by a user and should be persisted in a Node, the Computer should actually delegate to Node instead of trying to synchronize its state.

See JENKINS-XXXXX.

Testing done

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[Vlatombe]

The field offlineCause is only for technical offline cause now. temporarilyOfflineCause only exists in Node now.

[Vlatombe]

Retaining previous behaviour : if a temporary offline cause is defined, from outside PoV, it replaces the technical offline cause.

[jglick]

getNodeOrDie is probably wrong. Otherwise looks good.

[jglick]

I think this nullness check needs to be moved up into the callers, since some are just reading the status, not trying to set it, right?

[jglick]

For example this will throw an ISE from the whole api/json endpoint of a computer corresponding to a recently removed permanent agent, which is a legal situation.

Suggested change

	return getNodeOrDie().isTemporarilyOffline() || getChannel() == null;
	var node = getNode();
	return node != null && node.isTemporarilyOffline() || getChannel() == null;

getNodeOrDie could probably just be removed, and callers of getNode should decide how best to handle nullness; even for an offline setter, an ISE seems inappropriate—either just ignore the request (harmless, soon to be meaningless) or return a 404 or whatever.