Improve ipam status message before and after pending consensus

[bboreham]

Use the presence of a ticker object to determine whether we are actually waiting for consensus.
I also pulled the 'pending' bit out of the if-I-have-a-ring clause, because it is of interest either way.

Status before anything has happened now looks like this:

Allocator subnet 10.3.0.0/16

and after one weave run:

Allocator subnet 10.3.0.0/16 awaiting consensus: Nodes known: 1, Quorum size: 5
Pending requests for Allocate for 8450fe9a4cf18299cc745d32b8d052ac9e6cb3f069710a58eaff19abed78d607, 

Fixes #787

[rade]

I think the output when there are pending requests should look like this:

Allocator subnet 10.3.0.0/16 awaiting consensus: Nodes known: 1, Quorum size: 5
Pending requests:
  Allocate for 8450fe9a4cf18299cc745d32b8d052ac9e6cb3f069710a58eaff19abed78d607
  Allocate for ...