Mailbox clients for communicating with our own mailbox and contacts' mailboxes are managed by a singleton mailbox client manager (#2228). This ticket covers the client for communicating with our own mailbox. Some code will be shared with the client for communicating with a contact's mailbox (#2289). The shared code is covered by #2229. The client's connectivity check will use #2170. * At any given time the client has zero or more contacts assigned for upload and zero or more contacts assigned for download * Each contact assigned for upload has its own upload worker * The client has a single download worker that's shared by all contacts assigned for download * The client also has a single contact list worker When the client is created: * Create and start the contact list worker (#2294) When the client is destroyed: * Do all the stuff the superclass does when it's destroyed * Destroy the contact list worker When a contact is assigned for upload: * Create and start the contact's upload worker (#2291) When a contact is deassigned for upload: * Destroy the contact's upload worker When a contact is assigned for download: * If this is the only contact assigned for download: * Create and start the shared download worker (#2293) When a contact is deassigned for download: * If there are no more contacts assigned for download: * Destroy the shared download worker