cpu.sh is long-running routine which breaks status line refresh behavior

See the background for this issue here: https://github.com/samoshkin/tmux-config/issues/4

In a short, cpu.sh is long running process, which takes $status-refresh-interval-1 seconds to complete. This cause various issues regarding status line refresh behavior:

initially, when you start tmux, there is no CPU metric available, so you see cmd ~/.tmux/plugins/sysstat/scripts/cpu.sh not ready message.
when cpu.sh completes after 4 seconds, tmux refreshes all widgets in status line (not only cpu). This cause excessive spawns of other widget's scripts (mem, loadavg, swap, any other like tmux-status-online), so refresh behavior of status bar doesn't follow status-refresh-interval settings any more.

The idea for solution is to split cpu.sh logic into:

cpu collect routine, which blocks for $interval seconds and runs in background
cpu report/print routine, which just examines some location (file) for last CPU metric value and returns it to tmux to display in a status line.

Redesigned cpu.sh logic, split it into 2 files:

cpu.sh, called by status line, just examines temporary file for CPU usage metric value. Plus, starts CPU collect process, if not running yet. cpu.sh returns immediately, same to mem.sh and loadavg.sh.
cpu_collect.sh, long-running process, which is run on background, collect CPU usage samples at regular $status-refresh-interval using vmstat or iostat or top and stores last value in temp file (examined by cpu.sh later).

Now, from a sysdig trace, we can ensure, that cpu.sh, mem.sh, loadavg.sh are run strictly once per $status-refresh-interval.

19:43:46 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/mem.sh 13638
19:43:46 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/loadavg.sh 13639
19:43:46 | /home/alexeys/.tmux/plugins/tmux-online-status/scripts/online_status_icon.sh 13641
19:43:46 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/cpu.sh 13637

19:43:51 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/mem.sh 13786
19:43:51 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/loadavg.sh 13787
19:43:51 | /home/alexeys/.tmux/plugins/tmux-online-status/scripts/online_status_icon.sh 13789
19:43:51 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/cpu.sh 13785

19:43:56 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/mem.sh 13934
19:43:56 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/loadavg.sh 13935
19:43:56 | /home/alexeys/.tmux/plugins/tmux-online-status/scripts/online_status_icon.sh 13937
19:43:56 | /home/alexeys/.tmux/plugins/tmux-plugin-sysstat/scripts/cpu.sh 13933

As a side effect, when you start tmux session, you will immediately get average CPU usage since the system bootstrap in your status line, which is replaced by CPU average of each next interval.

samoshkin / tmux-plugin-sysstat

cpu.sh is long-running routine which breaks status line refresh behavior #4