[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

[massive store] Drop dead code This function is never used

[massive store] Reintroduce methods that are necessary to properly handle master/slave configuration Related to #15538303

[massive store] Follow configuration of the metadata generator Don't drop constraints and indexes for tables that are ignored by the metadata generator given to the store. One may now easily disable insertion of e.g. created_by / owned_by by removing them from the MetadataGenerator.META_RELATIONS set, in which case indexes for associated table won't be removed by the massive store.