server: Prefer 3 min mix capable outbound peers.

[davecgh]

The current logic attempts to maintain at least one mix capable outbound peer to ensure better connectivity among the mix capable peers. It works as intended, however, only having a single mix capable peer means there will be delays if that peer goes offline until a replacement is found. A possible consequence is that any mix clients might potentially get interrupted if that peer goes offline and a replacement isn't found quickly enough.

Attempting to maintain a minimum of three mix capable outbound peers will significantly reduce the likelihood of having periods without any mix capable peers.

This updates the logic accordingly.

[DBG] SRVR: Rejecting outbound peer <redacted> with protocol version 9 in favor of a peer with minimum version 10 (have: 0, target: 3)
...
[DBG] SRVR: Rejecting outbound peer <redacted> with protocol version 9 in favor of a peer with minimum version 10 (have: 2, target: 3)
...

$ dcrctl getpeerinfo | jq 'map(select(.version>=10)) | length'
3

[davecgh]

I shortened the debug message a bit per the edited PR description.