walk: Flush stdout in batches

[tavianator]

The previous behaviour was designed to mimic the output buffering of
typical UNIX tools: line-buffered if stdout is a TTY, and fully-buffered
otherwise. More precicely, when printing to a terminal, fd would flush
explicitly after printing any buffered results, then flush after every
single result once streaming mode started. When not printing to a
terminal, fd never explicitly flushed, so writes would only happen as
the BufWriter filled up.

The new behaviour actually unifies the TTY and non-TTY cases: we flush
after printing the buffered results, then once we start streaming, we
flush after each batch, but only when the channel is empty. This
provides a good balance: if the channel is empty, the receiver thread
might as well flush before it goes to sleep waiting for more results.
If the channel is non-empty, we might as well process those results
before deciding to flush.

For TTYs, this should improve performance by consolidating write() calls
without sacrificing interactivity. For non-TTYs, we'll be flushing more
often, but only when the receiver would otherwise have nothing to do,
thus improving interactivity without sacrificing performance. This is
particularly handy when fd is piped into another command (such as head
or grep): with the old behaviour, fd could wait for the whole traversal
to finish before printing anything. With the new behaviour, fd will
print those results soon after they are received.

Fixes #1313.

[tavianator]

Benchmarks:

First of all, here's the purpose of the PR:

tavianator@tachyon $ fd-master title_t00.mkv ~ | time head -n1
.../title_t00.mkv
head -n1  0.00s user 0.00s system 0% cpu 1.435 total
tavianator@tachyon $ fd-flush title_t00.mkv ~ | time head -n1
.../title_t00.mkv
head -n1  0.00s user 0.00s system 1% cpu 0.104 total

--output=null

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
fd-master -u --search-path ~/code/bfs/bench/corpus/chromium	296.7 ± 5.2	288.2	305.5	1.00
fd-flush -u --search-path ~/code/bfs/bench/corpus/chromium	296.8 ± 6.8	287.1	307.9	1.00 ± 0.03

--output=pipe

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
fd-master -u --search-path ~/code/bfs/bench/corpus/chromium	353.3 ± 19.3	320.7	381.1	1.01 ± 0.07
fd-flush -u --search-path ~/code/bfs/bench/corpus/chromium	351.1 ± 14.4	329.2	368.6	1.00

--output=inherit (aka a TTY)

I sent the output to an inactive tmux pane so it wouldn't depend on my actual terminal emulator or my ssh connection.

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
fd-master -u --search-path ~/code/bfs/bench/corpus/rust	609.1 ± 12.6	592.5	635.8	1.04 ± 0.03
fd-flush -u --search-path ~/code/bfs/bench/corpus/rust	588.2 ± 14.9	559.3	604.3	1.00

[sharkdp]

Thank you very much!