conversation_module: fix remove on replay

When a trust request is confirmed with an old conversationId, new
messages will be transferred and the conversation will be replaced
to avoid duplicates.
However, because erasePending() wasn't used after a lock, the
removal was failing with "operation not permitted" causing a duplicate
to stay.
A test is added to avoid any further regression.

Change-Id: Ic0fd960005a064ddd7b410f014f67d9125e50446