Avoid repeated lookups in mapOrphanTransactions and mapOrphanTransactionsByPrev

Rebased-From: 89d91f6
Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Fix crashing bug caused by orphan(s) with duplicate prevout.hash

Rebased-From: def2fdb
Rebased-By: Wladimir J. van der Laan
Limit CNode::mapAskFor

Tighten resource constraints on CNode.

Rebased-From: d4168c8
Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>
Avoid returning many "inv" orphans

Rebased-From: 540ac45
Rebased-By: Wladimir J. van der Laan <laanwj@gmail.com>