Mail: Persons tab creates huge number of server requests

[benbucksch]

Reproduction

1. Go to Mail | Vertical view
2. Select the Persons tab
3. Select a person
4. Unselect it
5. Select one or multiple other persons

Actual result

• App creates thousands of server requests to the mail server. Happens in IMAP (seen by Nick) and Exchange protocols (seen by Ben many months ago), so it's protocol independent.
• Server freaks out and blocks the user. Might cause Mail, Sent folder, Invalid credentials (Failure) #350
• No other user observable problem, other than the app being very slow and erratic

Expected result

• Only very few network requests
• No looping
• No insane (unnecessarily much) processing

[benbucksch]

To add logging:

• For IMAP, you can add a console.log() in IMAPFolder.runCommand(). Even IMAPEMail.download() etc goes through that. You won't have any info about the command being run, but at least you know that we contact the server.
• For HTTP requests (e.g. Exchange protocols), you can use backend.ts line 150:
  ```// console.log("Calling server", "input", input, "options", options, "defaults", defaultOptions);` ``