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