No Debug LTO

Don't enable LTO in debug mode, it makes debugging with source code
correspondences impossible.
CBXADProxy File Descriptor reduction

CBXADProxy now keeps at most 128 files cached. Keeping archives open
consumes file descriptors, which are limited (usually 256 per process).
Since opening files is not free, caching should help with repeated access
such as when getting multiple images from the same archive in a row.
CBXADArchiveParserProxy

More encapsulation for XADArchiveParser. The new proxy class handles
locking and parsing, if required. Some archive formats need to be
parsed before using them, so reopening them from disk requires reparsing.
The XADArchiveParser class does not expose wether this is needed. Locking
of the base archive parser is needed since reading changes its state in
a non thread-safe manner.