Reuse IMAP client for \OCA\Mail\Service\Sync\ImapToDbSynchronizer::syncAccount

[ChristophWurst]

Is your feature request related to a problem? Please describe.

The method \OCA\Mail\Service\Sync\ImapToDbSynchronizer::syncAccount iterates over an account's mailboxes and synchronizes them. For each mailbox we create a new client, which causes a new connection, and slows down the process.

Describe the solution you'd like

• Create the client instance in \OCA\Mail\Service\Sync\ImapToDbSynchronizer::syncAccount
• Adjust \OCA\Mail\Service\Sync\ImapToDbSynchronizer::sync to take the client as parameter instead of creating it via the factory

Describe alternatives you've considered

Cache connections in the IMAP client factory: d68749d. Reverted via a23e823 because it caused too many open connections.

Additional context

No response