cubicweb #4618949 [db-dump portable] lower the block size to avoid memory errors [validation pending]

Entities with fat .data attributes can eat memory quite fast.
The blocksize should be lower by default and also tunable.

prioritynormal
typebug
done in3.17.18
load0.100
load left0.000
closed by#7d9256aab337 [source/native/backup_restore] have a unique tunable blocksize for the dump phase
patch[source/native/backup_restore] have a unique tunable blocksize for the dump phase [applied]