Allow MailboxManager to supply RFC 2342 "IMAP4 Namespace" meta data

RFC 2342 "IMAP4 Namespace" is currently hardcoded to return only a single Personal Namespace with a hard coded, default hierarchy deliminated.

The specification allows multiple namespaces of the following types:

• Personal Namespace
• Other Users' Namespace
• Shared Namespaces

each of which may support separate hierarchy deliminators.

This meta data may vary with the user. Clients typically query only once (at the start of a session) and it should be considered fixed for the duration of the session. Any changes in permissions should be dealt with by applying appropriate security dynamically.

The current NAMESPACE implementation should be improved to allow the Mailbox implementation to supply this information.