Create healthyNetworkFraction metric that uses wall clock time

The metric `healthyNetworkFraction` currently works off of consensus time. This has two implications:

- honest nodes never send false alarms that the network is down. If a node has connectivity problems, it will stop reporting new values for this metric.
- if consensus halts, this metric stops reporting new data. It's actually impossible for this metric to report values that are 2/3 or lower.

To augment this, create a new metric called `presumedHealthyNetworkFraction` that uses wall clock time. It should compute the fraction of nodes (with consensus weight) that have created an event in the last 10 seconds as measured by wall clocks.

- if the network halts, all honest nodes will report 0 for this metric
- if an individual node becomes partitioned away from the network, it may falsely report 0 even if the rest of the network is alive and healthy