Use a singleton Azure Cosmos DB client for the lifetime of your application

[travis-leith]

The official performance guidance (at least for v3 SDK) is to keep a single client for the lifetime of the app.

I think this library should either implement the singleton pattern, or provide an argument to execAsync that takes in the client.

Unless, of course, this advice is no longer relevant for v4. I could not find a similar page for v4.

[travis-leith]

I'd like to submit a PR to address this but because of an Ionide issue you are already aware of, I cannot get it to build with the Analyzer. Perhaps this should wait until Ionide upgrade their dependencies?

[aaronpowell]

I assume it's the same guidance for v4, I've reached out to the team for clarity

[aaronpowell]

This will be coming in v1, check out #54 for more about v1