bibdocfile: support for X-Sendfile

• Added support for X-Sendfile HTTP header, used by Apache's mod_xsendfile or by lighttpd web server, in order to delegate back to the web server the job of streaming full-text files, liberating Invenio worker processes. Introduced new CFG_BIBDOCFILE_USE_XSENDFILE to toggle this feature.

• Introduced CFG_BIBDOCFILE_MD5_CHECK_PROBABILITY to be able to set the MD5 checking action frequency when streaming full-text files. (Before it was hardcoded to 0.25.)