Parameter to specify maximum buffer size for reading/writing files

To small buffers can result in long transfer times.
Add a new parameter buffer_size (default: 32768).

Trilead has built-in limits:
Javadoc comment of read: "Read bytes from a file. No more than 32768 bytes may be read at once."
Javadoc comment of write: "Write bytes to a file. If len > 32768, then the write operation will be split into multiple writes."