Cummulative stream size calculation

[gjedlicska]

The (semi) user facing issue this is to address is, the slow loading admin streams view or any other view where we need to calculate the disk size the stream consumes.

Since we only allow deleting stream objects from the database when we delete the stream, we can easily calculate the stream size just by incrementing a size number when inserting new objects to the stream.

The only tricky part is, to run a migration that seeds the current size values of existing streams.