Configure arc size clamping per agent+space

[mattyg]

AC:

• When a space is instantiated by an agent they pass in a config
• The config includes an arc size clamping setting
• If multiple agents in the same space pass in conflicting arc size clamping config, the applied config is the union of those

The end-goal of this is to enable holochain to specify arc clamping per-cell so that clones of a cell can have different arc clamping config.

[neonphog]

I don't know how this is going to shake out in the holochain integration, but luckily we can set that aside for a moment.

K2 is going to work a little differently in that everyone will start with a zero arc (legacy was supposed to evolve into this at some point too, it just never did). So "clamping" to a full arc would be incorrect and we will not implement that.

We can still clamp to zero arcs, or maybe we want to have some kind of enum on the LocalAgent for:

pub enum ArcStrategy {
  /// The arc sizing algorithm will try to move
  /// the arc size toward this value.
  Target(u32),
  
  /// The arc sizing algorithm will take into acount
  /// this node's desire to be this factor greater
  /// or less than the ideal arc size determined by the DHT
  Factor(f64),
}

The gossip arcs already are the union of all current arcs claimed by local agents within the same space, so we don't have to specify that explicitly again.