txorphanage: Move functions and data into class

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.

test/functional/p2p_invalid_tx.py

@@ -154,7 +154,7 @@ class InvalidTxRequestTest(BitcoinTestFramework):
             orphan_tx_pool[i].vin.append(CTxIn(outpoint=COutPoint(i, 333)))
             orphan_tx_pool[i].vout.append(CTxOut(nValue=11 * COIN, scriptPubKey=SCRIPT_PUB_KEY_OP_TRUE))
 
-        with node.assert_debug_log(['mapOrphan overflow, removed 1 tx']):
+        with node.assert_debug_log(['orphanage overflow, removed 1 tx']):
             node.p2ps[0].send_txs_and_test(orphan_tx_pool, node, success=False)
 
         rejected_parent = CTransaction()