Handle IMAP disconnects more explicitly

[ChristophWurst]

This changes the IMAP client usages to be used as a resource that is
freed after finished use. Previously we just memoized connections to the
same account to lower the number of connections, but that has still
shown to cause too many open connections during tests but possibly also
in production.

• Move IMAP client usage into try-(catch-)finally patterns
• Drop the IMAP client factory memoization
• Open follow-up ticket to diagnose if we have potential reconnects because of this change. If we previously used the memoized connection in two sections of a code there is now a little overhead. It might not be worth the optimization but should be checked nevertheless. IMAP connection reuse #6175

[miaulalala]

mail/lib/Account.php

Lines 185 to 190 in 689ef95

	public function getMailbox($folderId) {
	return new Mailbox(
	$this->getImapConnection(),
	new Horde_Imap_Client_Mailbox($folderId)
	);
	}

Is still using the deprecated getImapConnection method.

Refctor?

[ChristophWurst]

\OCA\Mail\Account::getMailbox is deprecated itself. I wouldn't refactor this anymore but try to get rid of those usages.