[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.

[datafeed] attempt to acquire synchronization lock even when force is given instead of the implementation in e717da3dc164, raise an error if the lock is already grabbed and catch this error in the caller. See discussion on https://www.cubicweb.org/revision/10790765 Closes #10451635

[schema] Cache RQL expressions on schema loading Add a mapping rql string to rql statement Deserializing schema is ~36% faster with a lot (> 4000) expressions like those generated by cubicweb-container.