[server] Improve massive deletion of entities sqlserver is limited on the "IN" array size and produce an error "The query processor ran out of internal resources and could not produce a query plan". The exact limitation is unclear but can occur with > 10000 item in the array. See https://support.microsoft.com/en-us/kb/288095 Now we process the deletion by batch of 10000, some quick benchmarks show that there is no performance impact: delete_in(50000): 46.765182972 delete_in_batch(50000): 46.3242430687 delete_in(100000): 62.2969429493 delete_in_batch(100000): 61.8402349949

[hooks] Add a bunch of tests for session synchronization And change unicode into six.text_type in authobjs.py spotted by new tests.

[entities] Drop very old backward compat from erudi time (even the standard datetime type didn't exist at that time)

pep8 + docstrings and comments improvments

[server] some pep8 in rql2sql

[dataimport] Deprecate SQLGenObjectStore Closes #10338709.