Closes #450

Closes #441

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.

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

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

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.

* force decline when two of our own identities are introduced to each
  other
* throw away introduction requests to the same identity
  (impossible to trigger from UI)

Closes #284

The database's transaction lock is reentrant, meaning that a thread that's already holding the lock can acquire it again. This would allow a thread that already has a transaction in progress to start another transaction, which could cause transaction isolation issues and/or lock timeouts on the database's internal locks.

Check that the current thread isn't already holding the lock when starting a transaction.