Flesh out addrmanger with some basic functionality.

Use it to add multiple peer support. We try and keep 8 outbound peers
active at all times.

This address manager is not as complete as the one in bitcoind yet, but
additional functionality is being worked on.

We currently handle (in a similar manner to bitcoind):

- biasing between new and already tried addresses based on number of connected
  peers.
- rejection of non-default ports until desparate
- address selection probabilities based on last successful connection and number
  of failures.
- routability checks based on known unroutable subnets.
- only connecting to each network `group' once at any one time.

We currently lack support for:
- tor ``addresses'' (an .onion address encoded in 64 bytes of ip address)
- full state save and restore (we just save a json with the list of known
  addresses in it)
- multiple buckets for new and tried addresses selected by a hash of address and
 source.  The current algorithm functions the same as bitcoind would with only
 one bucket for new and tried (making the address cache rather smaller than it
 otherwise would be).