bees: dynamic thread pool size based on system load average

Add -g / --loadavg-target parameter to track system load and add or
remove bees worker threads dynamically to keep system load close to the
loadavg target.  Thread count may vary from zero to the maximum
specified by -c or -C, and is adjusted every 5 seconds.

This is better than implementing a similar load average scheme from
outside of the process (though that is still possible) because the
in-process load tracker does not disrupt the performance timing feedback
mechanisms as a freezer cgroup or SIGSTOP would when controlling bees
from outside.  The internal load average tracker can also adjust the
number of active threads while an external tracker can only choose from
the maximum or zero.

Also fix a bug where a Task could deadlock waiting for itself to exit
if it tries to insert a new Task after the number of worker threads has
been set to zero.

Also correct usage message for --scan-mode (values are 0..2) since
we are touching adjacent lines anyway.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

README.md

@@ -535,13 +535,19 @@ Command Line Options
 --------------------
 
 * --thread-count (-c) COUNT
-  * Specify number of worker threads for scanning.  Overrides --thread-factor (-C)
-    and default/autodetected values.
+  * Specify maximum number of worker threads for scanning.  Overrides
+    --thread-factor (-C) and default/autodetected values.
 * --thread-factor (-C) FACTOR
   * Specify ratio of worker threads to CPU cores.  Overridden by --thread-count (-c).
     Default is 1.0, i.e. 1 worker thread per detected CPU.  Use values
     below 1.0 to leave some cores idle, or above 1.0 if there are more
     disks than CPUs in the filesystem.
+* --loadavg-target (-g) LOADAVG
+  * Specify load average target for dynamic worker threads.
+    Threads will be started or stopped subject to the upper limit imposed
+    by thread-factor and thread-count until the load average is within
+    +/- 0.5 of LOADAVG.
+
 * --scan-mode (-m) MODE
   * Specify extent scanning algorithm.  Default mode is 0.
     _EXPERIMENTAL_ feature that may go away.
@@ -552,6 +558,7 @@ Command Line Options
       on non-spinning media when subvols are unrelated.
     * Mode 2: scan all extents from one subvol at a time.  Good sequential
       read performance for spinning media.  Maximizes temporary space usage.
+
 * --timestamps (-t)
   * Enable timestamps in log output.
 * --no-timestamps (-T)