conversation: fix bug related to use of shut down swarm manager

The SwarmManager class uses the 'isShutdown_' variable to keep track of
whether it has been shut down or not. This variable needs to be reset to
false if we want to restart using a SwarmManager after it was shut down,
which we sometimes do when bootstrapping a conversation.

This patch fixes a failure in
ConversationRequestTest::testBanContactRestartAccount that was
introduced by commit eb4213d3.
GitLab: #1002

Change-Id: If5d295cbae4edd9e2292d7464e2eb2939b05eedb