Merge #14906: refactor: Make explicit CMutableTransaction -> CTransaction conversion.

b301950df32443e358bc22ca22c6f9ac09d18219  Made expicit constructor CTransaction(const CMutableTransaction &tx). (lucash-dev)
faf29dd019efef4b05e8e78885926764134d9c04  Minimal changes to comply with explicit CMutableTransaction -> CTranaction conversion. (lucash-dev)

Pull request description:

  This PR is re-submission of #14156, which was automatically closed by github (glitch?)

  Original description:

  This PR makes explicit the now implicit conversion constructor `CTransaction(const CMutableTransaction&)` in `transaction.h`.
  Minimal changes were made elsewhere to make the code compilable. I'll follow up with other PRs to address individually refactoring functions that should have a `CMutableTransaction` version, or where a `CTransaction` should be reused.

  The rationale for this change is:

   - Conversion constructors should not be explicit unless there's a strong reason for it (in the opinion of, for example, https://google.github.io/styleguide/cppguide.html, and https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ro-conversion. Let me know your take on this).
   - This particular conversion is very costly -- it implies a serialization plus hash of the transaction.
   - Even though `CTransaction` and `CMutableTransaction` represent the same data, they have very different use cases and performance properties.
   - Making it explicit allows for easier reasoning of performance trade-offs.
   - There has been previous performance issues caused by unneeded use of this implicit conversion.
   - This PR creates a map for places to look for possible refactoring and performance gains (this benefit still holds if the PR is not merged).

Tree-SHA512: 2427462e7211b5ffc7299dae17339d27f8c43266e0895690fda49a83c72751bd2489d4471b3993075a18f3fef25d741243e5010b2f49aeef4a9688b30b6d0631

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