Merge #11293: Deduplicate CMerkleBlock construction code, add test coverage

Merge #11293: Deduplicate CMerkleBlock construction code, add test coverage

46ce223d1 Add tests for CMerkleBlock usage with txids specified (James O'Beirne)
5ab586f90 Consolidate CMerkleBlock constructor into a single method (James O'Beirne)

Pull request description:

  What started as a simple task to add test coverage ended up giving way to a light refactoring. This consolidates the mostly-identical `CMerkleBlock` constructors into one (using C++11 constructor delegation) and adds coverage for the by-txids construction case.

  ### Before

  ![selection_006](https://user-images.githubusercontent.com/73197/30242104-0f381fe4-9545-11e7-9617-83b87fce0456.png)

  ### After

  ![selection_008](https://user-images.githubusercontent.com/73197/30242107-1425dfaa-9545-11e7-9e6b-2c3432517dd1.png)

Tree-SHA512: eed84ed3e8bfc43473077b575c8252759a857e37275e4b36ca7cc2c17a65895e5f494bfd9d4aeab09fc6e98fc6a9c641ac7ecc0ddbeefe01a9e4308e7909e529