Sequence operations to credentials and clean up better when setting

[TylerLeonhardt]

Fixes #130893

Although this one has been challenging to reproduce, my suspicion is that users were hitting the following behavior:

• multiple windows would open due to a VS Code update
• multiple windows were now writing to SecretStorage
• on Windows we have to chunk these writes into multiple writes, but we weren't sequentializing these writes so these two async operations writing to the same place asynchronously would cause the Cred Man to get into a bad state
• User reloads and we read from secrets with chunks from two different SecretStorage#store() calls so we fail to parse the secrets
• User is asked to sign in again
• Additionally, those old chunks are left behind because we don't delete secrets when we fail to parse
• Eventually a user hits a limit of Cred Man for the number of secrets in it which gets them in a worse state

So with this fix, we will:

• Delete the old secret before writing the new one (in case they have secret there that is not the same number of chunks)
• Make sure every write/delete/get is handled sequentially so that these operations don't cause the secret store to be in a bad state