Never attempt to read more than in 512kB blocks

Also increase the threshold at which we increase the read block size.
512kB ought to be enough to read a file smoothly (famous last words)

This reduces the amount of unnecessary verbosity. When watching a SD 3Mbit/s DVB stream, it will take over a second to get 1MB of data, leading to a warning entry in the log for every single read.

Refs #10428