conversation: fix invalid message status cache

The Conversation::loadMessages2 function uses the information cached in
both loadedHistory_ and memberToStatus, so it's important for the two to
always remain consistent with each other. This patch fixes two issues
that could cause them to diverge:

1) the Conversation::clearCache function currently resets
   loadedHistory_, but not memberToStatus;

2) Conversation::Impl::loadMessage2 doesn't modify loadedHistory_ but
   sometimes resets memberToStatus.

GitLab: #1087
Change-Id: I21caedea996d5ad66b695a0f130cd473c5d5ddc4