interface and a factory for that.

added a cache to the IdentityManager, changed its signature, modified when and where the author is stored

made the author creation single-threaded again in the LifecycleManager, removed redundant code

and turn them into a regular string.

Also removes support for anonymous forum posts.
Closes #698

Also fixed a couple of bugs.

This way the forum and private group client do not need to keep track of
message timestamps themselves and do not need to interact with
post/message factories.

Fixes #350

This is also lays the groundwork for #384