Update locking and caching for OpenIdConnectCachingSecurityTokenProvider

[mderriey]

Update locking and caching for OpenIdConnectCachingSecurityTokenProvider

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Update locking and caching for OpenIdConnectCachingSecurityTokenProvider

Description

This is a proposed alternative to #3118.

Changes:

1. Keep a lightweight write lock through Interlocked.Exchange to ensure a single thread calls ConfigurationManager.GetConfigurationAsync at a time.
2. Threads will not wait to acquire a write lock anymore, and will return existing data if they can't.
3. Remove read lock on the assumption that that variable assignments and reads are atomic operations for reference types (as per Update locking and caching for OpenIdConnectCachingSecurityTokenProvider #3118 (comment)).

Thanks for offering me to open a PR for this.

Please get as many eyes as you can on this to validate my assumptions. I don't have much confidence in my multi-threading, locking, and synchronization abilities.

Also, feel free to take over that PR to adjust to your coding preferences; one thing I noticed is the code base seems to prefer using Interlocked.CompareExchange over Interlocked.Exchange.
Essentially, this is now yours, although I'll definitely keep an eye on it and answer questions where I can.

Fixes #3078