Keep track of recently rejected transactions.
Keep track of recently rejected transactions.

Upstream commit cab74feb59e5b3aaa20e83a6ea8fdd69e7638c15.

jcmincke/dcrd,jcmincke/dcrd,decred/dcrd,jrick/dcrd,jcmincke/dcrd,jrick/dcrd,jrick/btcd,jrick/btcd,decred/dcrd,jrick/btcd