swarm: fix racecondition with acceptTrustRequest

AcceptTrustRequest should update conversation's module to directly
return the syncing conversation. Moreover, configurationMutex_ should
be recursive to allow sync between trust and conversation's requests.

This fix ut_conversationRequest where the problem was that the
conversation's requests were updated after ContactAdded, causing
removeContact() to remove the conversation, but acceptTrustRequest
re-adds it during the operation.

Change-Id: If676dc58290f87c289a6e63eb9afa04d2cc25df5
GitLab: #659