crucible: progress: a progress tracker for worker queues

The task queue can become very large with many subvols, requiring hours
for the queue to clear.  'beescrawl.dat' saves in the meantime will save
the work currently scheduled, not the work currently completed.

Fix by tracking progress with ProgressTracker.  ProgressTracker::begin()
gives the last completed crawl position.  ProgressTracker::end() gives
the last scheduled crawl position.  begin() does not advance if there
is any item between begin() and end() is not yet completed.  In between
are crawled extents that are on the task queue but not yet processed.
The file 'beescrawl.dat' saves the begin() position while the extent
scanning task queue is fed from the end() position.

Also remove an unused method crawl_state_get() and repurpose the
operator<(BeesCrawlState) that nobody was using.

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

test/Makefile

@@ -5,6 +5,7 @@ PROGRAMS = \
 	limits \
 	path \
 	process \
+	progress \
 	task \
 
 all: test

test/progress.cc

@@ -0,0 +1,40 @@
+#include "tests.h"
+
+#include "crucible/progress.h"
+
+#include <cassert>
+
+#include <unistd.h>
+
+using namespace crucible;
+using namespace std;
+
+void
+test_progress()
+{
+	ProgressTracker<uint64_t> pt(123);
+	auto hold = pt.hold(234);
+	auto hold2 = pt.hold(345);
+	assert(pt.begin() == 123);
+	assert(pt.end() == 345);
+	auto hold3 = pt.hold(456);
+	assert(pt.begin() == 123);
+	assert(pt.end() == 456);
+	hold2.reset();
+	assert(pt.begin() == 123);
+	assert(pt.end() == 456);
+	hold.reset();
+	assert(pt.begin() == 345);
+	assert(pt.end() == 456);
+	hold3.reset();
+	assert(pt.begin() == 456);
+	assert(pt.end() == 456);
+}
+
+int
+main(int, char**)
+{
+	RUN_A_TEST(test_progress());
+
+	exit(EXIT_SUCCESS);
+}