cubicweb-elasticsearch #17090555 Avoid memory error when indexing large number of entities [validation pending]

index-in-es builds a single rset for each entity type before iterating on entities and feeding elasticsearch.

If there are several millions of entities for a given type, chances are it won't fit in memory.

done in0.6.1
load left0.000
closed by#56ae3afb23f2 limit memory usage of ``index-in-es`` command
patch[ccplugin] provide a --chunksize option to index-in-es [applied]limit memory usage of ``index-in-es`` command [applied]