txorphanage: Move functions and data into class

Summary:
Collects all the orphan handling globals into a single member var in
net_processing, and ensures access is encapuslated into the interface
functions. Also adds doxygen comments for methods.

This is a backport of core#21148 [11/14]
https://github.com/bitcoin/bitcoin/pull/21148/commits/6bd4963c069bfd0af420e8a3fb724c3b693a1e76

Depends on D11489

Test Plan:
With clang and debug:
ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11490