The connection manager should treat LAN connection specially

#### Version information:
go-ipfs version: 0.4.19

#### Type: enhancement

#### Description:

The connection manager currently treat indistinctly LAN and internet connection, even though they don't have the same cost, for all meaning of this term.

This means that a MDNS-enabled node might have its LAN connection dropped when the connection manager reach the high water mark, and be forced later to route through the internet for something that could be achievable locally, which is a bit silly. Also, nodes that are in the same local network usually are mode likely to require content from each others, be it because they are part of the same application or because users want to access the same content from different machine at home.

I'd like to suggest a smarter connection manager, that either:
- don't drop local connection
- treat specially local connections, with a dedicated low/high water mark