Add poll_reserve to ExExHandle

[onbjerg]

          no, we should just add the equivalent fo poll_reserve,

but can also do this separately, so no blocker here

Originally posted by @mattsse in #7340 (comment)

[mw2000]

I took a first pass at this. From what I understood from your (@onbjerg) and @mattsse conversation, we want to separate the poll_reserve logic out into a new function on the ExExHandle level that the manager can call. Was a relatively low effort on my end, so let me know if major chunks are missing or if I got the context wrong.

[shekhirin]

#7610 (review)