IOFile provides the same interface of old BufferedFile. It provides
extension points as pure virtual functions. Those functions are
implemented by 2 concrete subclasses: BufferedFile and GZipFile.
MINGW does not have vasprintf(), so we use _vscprintf() and
vsnprintf(). We want to reuse buffer, so for non-MINGW we use
vsnprintf() with retrying doubling buffer size if output is truncated.
Saved sessions may very large, as in hundreds and even thousands of
megabyte when dealing with large queues.
Add support to save and reload sessions to gzipped files, when libz is
available.
The session serializer will output gzipped contents when the file ends
with .gz, while the input file reader (UriListParser) will accept
whatever is thrown at it.