Sync should only download subscribed folders (LSUB)

[netandreus]

Describe the bug
As we discuss in #105 there is a problem with syncing Exchange mailboxes. There are many clients of Microsoft Office365 mail system, and I think it is important. Problem occurs when we sync folders like "Archive" e.t.c.

To Reproduce
Steps to reproduce the behavior:

1. Add Microsoft Office365 mail account.
2. Try to sync it
3. See errors

[19-Mar-20 11:54:04] DEBUG: No messages to delete in Archive
[19-Mar-20 11:54:04] ERROR: cannot read - connection closed?

Also when we try to sync folder "Calendar/United States holidays" in database added this messages:

**Message.subject**
Retrieval using the IMAP4 protocol failed for the following message: 367

**Message.text_plain**
The server couldn't retrieve the following message:

Subject: "Juneteenth"
From: "xxxx" ("/O=EXCHANGELABS/OU=EXCHANGE ADMINISTRATIVE GROUP (FYDIBOHF23SPDLT)/CN=RECIPIENTS/CN=D6FFC709A0FE4E2F8AD251C6DE637AAD-ADMIN")
Sent date: 9/21/2019 9:00:25 PM

Expected behavior
Correctly folders sync.

Possible solution
As we decide it should be mechanism to per-account select folders to sync. And this mechanism already exists - it is "folder subscriptions". But it has one disadvantage. Folders "INBOX" and "Outbox" by default are unsubscribed and Outlook client synced it ignoring subscriptions.

So we need to set mandatory_sync_folders parameter somewhere (for example in default.ini / local.ini).
It also needed parameter sync_only_subscribed_folders, which will be regulate behavior of this functionality. Default sync_only_subscribed_folders = false for current behavior.

Requirements
I have already done some work with required libraries. You just need to review it :)

• Add ability to load only subscribed folders in Mailbox. ParticleBits/zend-mail#5 (to develop branch)
• Mailbox->getFolders() Ability to load only subscribed folders ParticleBits/imap#4 (to master branch)

[netandreus]

I finish this work @mikegioia Please review my pull request #121