Protect localhost and block-relay-only peers from eviction

Onion peers are disadvantaged under our eviction criteria, so prevent eventual
eviction of them in the presence of contention for inbound slots by reserving
some slots for localhost peers (sorted by longest uptime).

Block-relay-only connections exist as a protection against eclipse attacks, by
creating a path for block propagation that may be unknown to adversaries.
Protect against inbound peer connection slot attacks from disconnecting such
peers by attempting to protect up to 8 peers that are not relaying transactions
but appear to be full-nodes, sorted by recency of last delivered block.

Thanks to gmaxwell for suggesting these strategies.

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