Reduce MAX_PEER_TX_ANNOUNCEMENTS for non-PF_RELAY peers

Maintaining up to 100000 INVs per peer is excessive, as that is far more
than fits in a typical mempool.

Also disable the "overload" penalty for PF_RELAY peers.

doc/release-notes-19988.md

@@ -0,0 +1,9 @@
+P2P changes
+-----------
+
+The size of the set of transactions that peers have announced and we consider
+for requests has been reduced from 100000 to 5000 (per peer), and further
+announcements will be ignored when that limit is reached. If you need to
+dump (very) large batches of transactions, exceptions can be made for trusted
+peers using the "relay" network permission. For localhost for example it can
+be enabled using the command line option `-whitelist=relay@127.0.0.1`.