Dispute coordinator - Recover disputes on startup (#3481)

* node/dispute-coordinator: Introduce resume capability

This commit introduces a resume capability for the
dispute coordinator subsystem. Specifically, this will allow
to recover data for disputes for which we have no local statements.

* node/dispute-coordinator: Add resume function to TestState and modify Harness

This commit modifies the TestHarness to return a TestState. We subsequently
define a resume function on TestState that allows to interrupt the test and
test specifically for behavior on startup of the subsystem.

* node/dispute-coordinator: Implement resume functionality

This commit implements the resume functionality for the subsystem.
In addition, we will forward any DisputeParticipation::Participate
message in order to ensure that disputes for which we do not have
local statements may be recovered in due time.

* Address Feedback

* Modify to run handle_leaf on first import

* Modify missing_local_statement logic

* node/dispute-coordinator: Add simple test to ensure we adequately
handle local_statements that are not missing.

* Add missing keystore tests