That fix a performance issue when typing a message. The sqlite database
was accessed each time a character was typed in order to mark all
preceeding messages as read. That was done even if all message was marked
as read.

This patch add cache for unread messages count. Then, the count is only carried out
at the first call of linphone_chat_room_get_unread_messages_count(). This function
is used in linphone_chat_room_mark_as_read() to prevent database from being accessed
if all messages are already marked as read.