* util.tty.log: read up to 100 lines if ready
Rework to read up to 100 lines from the captured stdin as long as data
is ready to be read immediately. Adds a helper function to poll with
`select` for ready data. This showed a roughly 5-10x perf improvement
for high-rate writes through the logger with relatively short lines.
* util.tty.log: Defer flushes to end of ready reads
Rather than flush per line, flush per set of reads. Since this is a
non-blocking loop, the total perceived wait is short.
* util.tty.log: only scan each line once, usually
Rather than always find all control characters then substitute them all,
use `subn` to count the number of control characters replaced. Only if
control characters exist find out what they are. This could be made
truly single pass with sub with a function, but it's a more intrusive
change and this got 99%ish of the performance improvement (roughly
another 2x in some cases).
* util.tty.log: remove check for `readable`
Python < 3 does not support a readable check on streams, should not be
necessary here since we control the only use and it's explicitly a
stream to be read.
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