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.

This is also lays the groundwork for #384

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

Closes #584, #586, #585

This was happening when the remote response arrives before the local
response is made and thus the local response needs to be send with the
ACK following. The problem was that we ACK was sent before the response
which is not allowed and resulted in the session being aborted by the
introducee. This was happening, because recursion is hard ;)

The fix is only restarting another protocol engine to send the ACK
after the first run has been completed.

An integration test was added to prevent such regression in the future
and to test this code path.

First problem was a race condition with message delivery and the second
one due to the fact that we no longer plan to allow adding of additional
blogs, so the test for that has simply been removed.

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.

The MAC and signature are not yet generated and verified.
This will happen in a later commit.

Comments and reblogs need to depend on the post they refer to.
Since message dependencies are limited to one group,
the post and also the comments need to be wrapped
when commented on or reblogged to another blog.

For this reason, in addition to comments, two new wrapping message types
are introduced. They retain all data of the original messages and allow
for reconstruction and signature verification.

This commit breaks backwards compatibility with old blog posts.
It removes the content type, title and parent ID from the post
message structure.

Closes #580

Only personal blogs from non-contacts can be removed.
This also adds integration tests that check if blogs can actually be removed.

Closes #579

Fixes #470

Closes #575

Closes #472

even after re-sharing an existing forum.

Closes #441

with each other at the same time.

Closes #388

and add integration tests that make sure that dependencies are handled
properly.

that handles message dependencies reported from clients.

The MessageValidatedEvent has been renamed into a MessageDeliveredEvent
since there were no real use cases for the former any more.

This adds a new table to the database to hold message dependencies.
It introduces two more message states: pending and delivered
The valid column in the database was renamed to state to better reflect
its new extended meaning.

The DatabaseComponent was extended with three methods for:
* adding dependencies
* getting dependencies of a message
* getting messages that depend on a message (dependents)
* getting messages to be delivered (by startup hook)
* getting pending messages to be possibly delivered (by startup hook)

In order to reflect the new states, things that were previously true for
VALID messages have been changed to now be true for DELIVERED messages.

Since pending messages should not be available to clients, many database
queries have been modified to only return results for delivered
messages.

All added methods and changes should come with updated unit tests.

Please note that the database version was bumped in this commit.

The new activity shows who you are sharing a forum with and who shares a
forum with you. It is accessible from the overflow menu when in a forum.

Closes #398

Closes #391

The code for creating forums in ForumManager was used by
ForumSharingManager and also needed by InviteeEngine.
This extracts it into its own class.

Closes #375

for introducer when both introducees have been deleted.

Closes #372