Merge #19053: refactor: replace CNode pointers by references within net_processing.{h,cpp}

8b3136bd307123a255b9166aa42a497a44bcce70 refactor: replace CNode pointers by references within net_processing.{h,cpp} (Sebastian Falbesoner)

Pull request description:

  This PR is inspired by a [recent code review comment](https://github.com/bitcoin/bitcoin/pull/19010#discussion_r426954791) on a PR that introduced new functions to the net_processing module. The point of the discussion was basically that whenever we pass something not by value (in the concrete example it was about `CNode*` and `CConnman*`) we should either use
  * a pointer (```CType*```) with null pointer check or
  * a reference (```CType&```)

  To keep things simple, this PR for a first approach
  * only tackles `CNode*` pointers
  * only within the net_processing module, i.e. no changes that would need adaption in other modules
  * keeps the names of the variables as they are

  I'm aware that PRs like this are kind of a PITA to review, but I think the code quality would increase if we get rid of pointers without nullptr check -- bloating up the code by adding all the missing checks would be the worse alternative, in my opinion.

  Possible follow-up PRs, in case this is received well:
  * replace CNode pointers by references for net module
  * replace CConnman pointers by references for net_processing module
  * ...

ACKs for top commit:
  MarcoFalke:
    ACK 8b3136bd307123a255b9166aa42a497a44bcce70 🔻
  practicalswift:
    ACK 8b3136bd307123a255b9166aa42a497a44bcce70

Tree-SHA512: 15b6a569ecdcb39341002b9f4e09b38ed4df077e3a3a50dfb1b72d98bdc9f9769c7c504f106456aa7748af8591af7bb836b72d46086df715ab116e4ac3224b3b

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