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

This reverts commit 43cba6537fae95cef0ff5ce6ba7352c52abcd947.
(cherry picked from commit bcfa0100d94e68df913f3a620da7315a556ccfeb)
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
(cherry picked from commit 1cfdfa3f56876eb20170f33e9592b5c7ffd05064)