Mismatch between peer count and connection count in trimming logic

[vyzo]

It has been observed in relays that the connection manager trims fewer connections than desired.
The explanation for this is a mismatch in the trim logic between peer and connection count.
We trigger the trim logic when the connection count exceeds the high water mark, but we trim using peer count - low water mark as target.
This breaks down in the presence of duplicate connections, and we may end up trimming less connections than desired.

[vyzo]

See also libp2p/go-libp2p-swarm#123