spider select handler: interim planning, design and implementation improvement

After the completion of MDEV-37111, before doing MDEV-37112 and MDEV-37113, there are a few questions to be answered, such as:

1. Do we need to implement a derived handler for spider to cover derived/views? Yes but can be a V2 task - see the design doc
2. Do we need to improve spider test coverage before the select handler can be added to the server code for real? If so, how much?

There are also numerous TODOs in the MDEV-37111 that is about the improvement of the implementation of spider sh that is not in the scope of MDEV-37112 or MDEV-37113. They need to be addressed too.

Other tasks:

• [X] support single-partition read

The design doc (attached) is updated to answer these questions, see esp. the new section "Post initial implementation evaluation and planning"