cta/CTA#139 cta af ls --vid XXXXX command times out if there are many files on a tape
Added the XrdCtaFile::m_listingArchiveFiles member variable and factorized XrdCtaFile::readFromCmdlineOutput() out from XrdCtaFile::read() so that in the future the XrdCtaFile::read() method will be able to treat the listing of archive files differently from other cta commands. Currently none of the cta commands are incrementally generating and streaming large result sets back to the cta command-line tool. All commands are generating the entire result in the constructor of XrdCtaFile. The result is streamed back to the cta command-line tool, but the result is generated in one single stepi before any streaming starts. The listing of archive files is being gradually modified to incrementaly generate the result in each call to XrdCtaFile::read(). This commit is one step in this larger modification.