This change will allow to pass message dependencies from the client
validators to the ValidationManager.

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

Closes #392

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

Added support for retrieving metadata that matches all key/value pairs in a query.

Prevent old forum sharing messages from causing spurious exceptions.

This allows the lifecycle manager to continue starting other services while plugins are starting, and allows the plugin manager to stop each plugin as soon as it has started.

It was possible that a malicious introducer sends new request with the
same session ID that was used previously and thus causing introducees to
have multiple states for the same session ID.
This commits prevents that from happening and adds an integration test
for that scenario.

Also if an introducee removes an introducer, all past session states
will be deleted from the database. For this, a test was added as well.

Closes #371
Closes #372

This commit replaces the old ForumSharingManagerImpl with a new one
which is based on state machines and the ProtocolEngine.

There is a SharerEngine and a InviteeEngine that take care of state
transitions, messages, events and trigger actions to be carried out by
the ForumSharingManagerImpl. This is all very similar to the
Introduction Client.

The general sharing paradigm has been changed from sharing as a state to
sharing as an action. Now the UI can allow users to invite contacts to
forums. The contacts can accept or decline the invitiation. Also, the
Forum Sharing Manger is notified when users leave a forum.

Closes #322

Methods for creating, adding and removing forums have been moved to the
`ForumManager`. In order to still handle removing forums properly, a
`RemoveForumHook` has been introduced.

Methods for sharing forums with all current and future contacts have
been removed along with the localGroup where this information was saved.

The `ShareForumActivity` now has the proper label.

The `SessionId` and the `ProtocolEngine` have been moved to the
`clients` package.

This addresses part of #322 and part of what has been discussed in #320.

This avoids wasted connection attempts to contacts that are in the process of being introduced.