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.
EraseOrphansFor was called both with and without g_cs_orphans held,
correct that so that it's always called with it already held.
LimitOrphanTxSize was always called with g_cs_orphans held, so
add annotations and don't lock it a second time.
