lib: Add event filtering to debug logging

lib: Add event API The events are hierarchical and the child events inherit the parents' fields. The events will be attached to "objects", which gives them some context. For example each mail user, mailbox and mail will have its own event. The events can also have a log prefix, so logging events to e.g. mail will always log a consistent prefix that identifies which mail is being logged about. The events will also be used for statistics. Typically this is done by logging a debug event, which contains fields that are interesting for statistics. The interesting events will have a name, which can be used to identify them when configuring which statistics are wanted to be tracked. It's possible to also track events that have no name, but that requires using the source code's filename:line as the event name. That of course may change between releases, so it's not very reliable.