[Event Hubs] Remove circular dependencies

[chradek]

When running npm build, rollup complains about the following circular dependencies:

(!) Circular dependency: dist-esm/src/eventHubClient.js -> dist-esm/src/connectionContext.js -> dist-esm/src/managementClient.js -> dist-esm/src/eventHubClient.js
(!) Circular dependency: dist-esm/src/eventHubClient.js -> dist-esm/src/sender.js -> dist-esm/src/eventHubSender.js -> dist-esm/src/eventHubClient.js
(!) Circular dependency: dist-esm/src/eventProcessor.js -> dist-esm/src/pumpManager.js -> dist-esm/src/eventProcessor.js
(!) Circular dependency: dist-esm/src/eventProcessor.js -> dist-esm/src/pumpManager.js -> dist-esm/src/partitionPump.js -> dist-esm/src/eventProcessor.js

We should update our code where possible to prevent circular dependencies.