Describe the purpose of your pull request

This PR makes some corrections and improvements around config services:

• Use monitonic clock (Stopwatch.GetTimestamp) instead of system clock (DateTime.UtcNow) for scheduling poll iterations in Auto Polling mode for improved precision and resistance to system clock adjustments (time sync, daylight saving, user initiated clock adjustments, etc.)
• Correct the local cache refresh behavior in offline mode: if the client uses an external cache, a synchronization should happen in every case where a fetch operation would happen in online mode. For example, in Auto Polling mode, the background polling loop should sync with the external cache even when it doesn't initiate config fetch operations.
• Change forceRefreshAsync to report failure and log a warning in offline mode only when the client is not configured to use an external cache. (Low impact behavioral BC.)
• Make the ClientReady event consistent with other SDKs in Auto Polling mode. When the client is offline, ClientReady is emitted as soon as the initial sync with the external cache completes. (Low impact behavioral BC.)
• Correct the intellisense docs for the ClientReady event and the WaitForReadyAsync method as the previous description was misleading.
• Eliminate possible race condition between initial and user-initiated cache synchronization (on asynchronous code path only).
• Make changes to also emit the ConfigChanged event when the local cache is updated as a result of synchronization with the external cache. (Low impact behavioral BC.)
• Raise configChanged once per config refresh operation, eliminate race conditions around cache write, and improve performance in high concurrency situations by deduplicating config refresh instead of config fetch operation only.
• Fix bug in HttpClientConfigFetcher (The W/ prefix should not be stripped from the received ETag because other SDKs don't do that).

Related issues (only if applicable)

https://trello.com/c/XZdBOLY1
https://trello.com/c/mU63054x

How to test? (only if applicable)

n/a

Security (only if applicable)

n/a

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.