Add optional Cluster instance limit

Suggested release note:

Added two new global config properties to help diagnose leaked Cluster instances:

`Cluster.maxAllowedInstances` is the number of connected Cluster instances that may exist at the same time. Calling `Cluster.connect` after this limit is reached will either fail or log a warning, depending on the value of the new `Cluster.failIfInstanceLimitReached` property. The default values log a warning if more than 1 Cluster is connected at a time.