[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
Philippe Pepiot <philippe.pepiot@logilab.fr> [Mon, 30 May 2016 17:41:12 +0200] rev 11344
[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.