Stricter handling of orphan transactions

Prevent denial-of-service attacks by banning
peers that send us invalid orphan transactions
and only storing orphan transactions given to
us by a peer while the peer is connected.
Store fewer orphan tx by default, add -maxorphantx option

There is no reason to store thousands of orphan transactions;
normally an orphan's parents will either be broadcast or
mined reasonably quickly.

This pull drops the maximum number of orphans from 10,000 down
to 100, and adds a command-line option (-maxorphantx) that is
just like -maxorphanblocks to override the default.