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

in group metadata to be able to speed up group listings.

Closes #584, #586, #585

Like other recursive operations on the dependency graph, this is
not done in a single transaction to prevent an attacker from creating
arbitrary large transactions.

So at startup, the `ValidationManager` finds and resumes any
unfinished operations, by looking for shared messages with unshared
dependencies.

This also adds unit tests to prevent regressions like this in the
future.

The docs say this can happen for a directory if there's an I/O error.

and use it for retreiving the status of an author faster.
Also add tests for both.

Before the introducee sends her ACK,
she derives a master key from the ephemeral shared secret as before.

Two nonces and a MAC key are then derived from the master key.
The local introducee signs one of the nonces and calculates a MAC
over her own identity public key, ephemeral public key,
transport properties and timestamp.
The local introducee includes the signature and MAC in her ACK.

On receiving the remote introducee's ACK,
the local introducee verifies the signature and MAC.
Should the verification fail, an ABORT is sent to the introducer and
the remote introducee that was added as inactive is deleted again.