[security] experimental rqlst cache for read security: to activate using a 'security-rqlst-cache' flag in transaction data when you'll issue a lot of identic queries with only kwargs varying

[read security] minor optimizations

[web] get rid of spurious '[Errno 2] No such file or directory' on load This was caused by can_do_pdf_conversion() which tries to use '/usr/bin/fop'. When fop was not available, an OSError was raised by suprocess.Popen. This changeset first checks for /usr/bin/fop presence, then tries to run fop. If an error occurs, log it instead of printing it. NOTE for later: why do we test explicitly for "/usr/bin/fop" rather than simply "fop" ?