Use $elemMatch operator for searching an announcement

[furkansimsekli]

Fetching all the announcements and looping through them to understand whether the given announcement is new or not might be memory intensive when there are A LOT of saved announcements. We can use $elemMatch for this task.

Also I'll see where else I can improve the performance with better queries.

Note: To reduce the number of requests that we send to database, we can use db.collection.bulkWrite()

[furkansimsekli]

I will be making changes to database design. Until then, this issue will be stalled. Probably #71 too.