Move towards a model where TxOrphanage is initialized with limits that
it remembers throughout its lifetime.
Remove the param. Limiting by number of unique orphans will be removed
in a later commit.
Now that -maxorphantx is gone, this does not change the node behavior.
The parameter is only used in tests.
Expiry is going away in a later commit.
This is only an RPC change. Behavior of the orphanage does not change.
Note that getorphantxs is marked experimental.
