If a least one email in a conversation that is in the base folder but
also in another is removed from the base folder, the conversation may
not be removed from the monitor despite possibly not having any email
in the base folder, since the email may not have been completely removed
from the conversation.

This was particulary being seen with GMail accounts where even single
message conversations were not disappering when trashed because the
converation's email was still in All Mail.

This fix does a few things: Avoids hitting the database when checking
a conversation still has email in the base folder, when it does check
only ensures that email are in the base folder, not *any* folder, and
updates ConversationSet::remove_all_emails_by_identifier to do this
check iternally, clean up its API and implementation and avoids having
to use out args calling it.