[massive store] isolate SQL statements that one may want to customize i.e. provide public methods for meta data insertions

[massive store] Store entities in temporary table as well * on some entity type is encountered by a slave, create a dedicated table for insertion of entities of this type by this slave, similarly to what is done for relation - this should lower changes of conflicts in master/slaves mode ; * delay drop of constraints and indexes to `finish` method, where copy from temporary tables to regular table is done ; * insertion of metadata is done by scanning temporary tables, which may be way shorter than theier associated regular table ; * drop drop_metadata_constraints with its constraint_dropped friend attribute, there are no more necessary since this is done once in the `finish`. Related to #15538303

[massive store] Drop useless check for empty buffer If something went wrong, an error will be raised.

[massive store] Docstring / comment cleanups

[massive store] Turn a runtime error into an assertion geting there is a bug.

[massive store] Delay metadata attributes and default values handling to flush_entities This may be done once for all there, instead of for each entity in prepare_insert_entity.

[massive store] Use a slave specific table for relation insertion in the massive store * attribute a random id to the store * add a column containing this id in the cwmassive_initialized table * separate generic `master_init` which create the cwmassive_initialized table from rtype specific initialization which must be done in each slave now * delay removal of table indexes to `finish` * get back relations from each table on `finish` Similar work still has to be done for entities insertion - which will allow more cleanup to the constraints handling which is still rough for now. Related to #15538303

[massive store] docstring and __init__ cleanup