WW-5270 Fix forwarding from Struts excluded URL

[kusalk]

Implements WW-5270

[kusalk]

My initial ThreadLocal based approach to reworking the request cleanup process was not functioning as intended. I've reverted to tracking filter recursion as a request attribute but have still retained the reworked logic.

This means:

• ActionContext is not cleared until the first StrutsPrepareFilter execution for that request has completed - this is irrespective of whether the ActionContext was generated there or not. This can occur when a Struts excluded URL forwards to a non-excluded one. Waiting until the ActionContext is cleared is required by SiteMesh.
• The MultiPart component of a request is only cleaned up in the filter execution in which it was initially wrapped. This is retaining the original behaviour as before. I believe if we try to cleanup at any other stage, it will fail as the instanceof check will fail - although I haven't investigated this closely.

The implementation is a little confusing at first glance (and was also why I favoured the ThreadLocal based approach) but it's the best I've got right now - open to suggestions.

[lukaszlenart]

Sorry, I totally missed this PR, LGTM 👍

[lukaszlenart]

@kusalk could you resolve conflicts? I see nothing but GH blocks merging this PR :\

[kusalk]

@lukaszlenart All good, should be sorted now. We shipped this change in Confluence 8.0.2 on Dec 22nd and there have been no reported issues.