Revert "Never attempt to read more than in 512kB blocks"

This reverts commit 43cba6537fae95cef0ff5ce6ba7352c52abcd947.
Never attempt to read more than in 512kB blocks

Recalculate the threshold and read block size when we've encountered EOF several times in a row.
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