Improve support for client sharing between Ensembles

[dougbrn]

Currently, the Ensemble has functionality to close a tied client during ending operations like exit and del. This is governed by the Ensemble.cleanup_client variable that is set when a client is tied to the Ensemble. This complicates the ability for multiple ensembles to share a given client, as one may attempt to close the client that is still in use by another ensemble. We should implement a more robust relationship between an ensemble and a client, such that multiple clients can use an ensemble without pre-emptive attempts to close it, but also eventually closing it when no Ensembles are left using it.

[wilsonbb]

Note that issues related to client sharing were observed in the flaky test failures in issue #328

Most of those failures were resolved by having mostly clientless unit tests (#376), but those failures are good examples of the kind of shared client issues we want to improve upon.