First pass at adding a read buffer to file_handle_cache.
Merge in default
If they ask to read more than the buffer size, do so.
Be a bit more systematic about reseting the read buffer.
Small refactor suggested by Matthias.
Remove a couple of TODOs, make the read buffer size configurable, and don't use the read buffer for the QI or msg store transform since they already read in decent sized chunks.
Merge bug26457