conversation: avoid potential deadlock

convModule()->bootstrap should not use configurationMutex_ while
locking conversations. Else we may have:
thread 1: configurationMutex_->conversationsMtx_
thread 2: conversationsMtx_->configurationMutex_
causing a deadlock

So, retrieve known devices before bootstrapping the conversation

Change-Id: Ib194edaa255b5ffde9e58ea7e0be0226943bb118