This prevents higher order orphans and other junk from
holding positions in the orphan map.  Parents delayed
twenty minutes are more are unlikely to ever arrive.

The freed space will improve the orphan matching success rate for
other transactions.

…ted.

An orphan whos parents were rejected is never going to connect, so there
is little utility in keeping it.

Orphans also helpfully tell us what we're missing, so go ahead and treat
it as INVed.

Although this increases node memory usage in the worst case by perhaps
30MB, the current behavior causes severe issues with dependent tx relay.

This is another violation of the one definition rule, as the type
for mapOrphanTransactionsByPrev did not match the one in
net_processing.cpp anymore. As it now depends on a custom Iterator,
it seems too much hassle to correctly expose it to the tests.
Instead, this commit just removes the one test it was referenced in.

This should (marginally) speed up validationinterface queue draining by avoiding a cs_main lock in one client.

The previous code was biased towards evicting transactions whose txid has
a larger gap (lexicographically) with the previous txid in the orphan pool.

In the logic for requesting missing parents of orphan transactions, parent
transactions with multiple outputs being spent by the given orphan were being
processed multiple times. Fix this by deduplicating the set of missing parent
txids first.

Accepting them up to the max supported size.