persistent cache resiliency

[SomkaPe]

Taking to account that file locks are not completely reliable on non Win platforms, It would be nice to implement following features to be race conditions resilient:

• Switch file cache accessor from non-atomic file writes to atomic file renaming (in order to prevent dirty reads + dirty writes - cache corruption)
• Retry logic on KeyChainAccessor to deal with race conditions

details can be found here -> https://github.com/AzureAD/microsoft-authentication-extensions-for-java/wiki/Race-conditions

[henrik-me]

@bgavrilMS : Java and soon Python has these improvements. We should ensure to properly plan for these in .NET.