conversation_module: fix clone from multiple devices

Multiple changes are included in this patch:
+ Remove pendingConversationFetchMtx_ and use conversationsMtx_
instead to avoid any potential weird state
+ Remove RepeatedTask causing two fetch operations to happen
+ Remove bugguy map access causing a fetch to happen even if
previously removed

A test for the problematic scenario is added to avoid any future
regression.

Change-Id: I9b5f93fc11f053a0552e5f34711130d5a5c22c05