Handling of Gmail HistoryId Expiration.

[bkaankose]

When the client keeps an expired historyId as delta sync token for Gmail, users are left with re-adding the account as a solution. This PR brings few improvements around this area:

• Expired historyId will now reset the local cache and start full synchronization. (as documented here)
• Added removing the local cache methods.
• Fixed an issue with account deletion where the actual mails were still stored on the disk.
• Notifying users when the cache reset was needed for the account.
• Skipping draft mapping for Gmail if there are no local drafts.