cubicweb #3192809 Customizable blocksize for db-portable restore [done]

database-independant-restore hardcodes the number of entities serialized in each block, cf:

http://hg.logilab.org/master/cubicweb/file/dd574929b414/server/sources/native.py#l1788

If entities hold large blobs of data, the block might not fit in memory so it would be nice to be able to customize its size on the command line.

prioritynormal
typebug
done in<not specified>
load0.250
load left0.000
closed by<not specified>