PLT-4618: Add test about very large task

Confirmed very large task (tasks whose data is ungzipped) makes DELETE
query degraded.

BEFORE (v0.8.45):

PerfectQueue::Backend::RDBCompatBackend
  #acquire
    very large jobs
D, [2016-03-22T18:35:03.678637 #24939] DEBUG -- : (0.000095s) SET @@wait_timeout = 2147483
D, [2016-03-22T18:35:03.678802 #24939] DEBUG -- : (0.000094s) SET SQL_AUTO_IS_NULL=0
D, [2016-03-22T18:35:03.680263 #24939] DEBUG -- : (0.001333s) BEGIN
D, [2016-03-22T18:35:03.680539 #24939] DEBUG -- : (0.000154s) SELECT GET_LOCK('test_queues', 60) locked
D, [2016-03-22T18:35:07.899606 #24939] DEBUG -- : (4.218841s) DELETE FROM `test_queues` WHERE timeout <= 1458639303 AND created_at IS NULL
D, [2016-03-22T18:35:07.899785 #24939] DEBUG -- : (0.000088s) COMMIT
D, [2016-03-22T18:35:07.900490 #24939] DEBUG -- : (0.000066s) SET @@wait_timeout = 2147483
D, [2016-03-22T18:35:07.900589 #24939] DEBUG -- : (0.000067s) SET SQL_AUTO_IS_NULL=0
D, [2016-03-22T18:35:07.900732 #24939] DEBUG -- : (0.000074s) BEGIN
D, [2016-03-22T18:35:07.900905 #24939] DEBUG -- : (0.000096s) SELECT GET_LOCK('test_queues', 60) locked
D, [2016-03-22T18:35:08.266121 #24939] DEBUG -- : (0.364732s) SELECT id, timeout, data, created_at, resource
FROM `test_queues`
WHERE timeout <= 1458639303 AND timeout <= 1458639303 AND created_at IS NOT NULL
ORDER BY timeout ASC
LIMIT 10

D, [2016-03-22T18:35:08.637478 #24939] DEBUG -- : (0.225528s) UPDATE `test_queues` SET timeout=1458639345 WHERE id IN ('4f52ab9c-f011-11e5-91d1-61143f034f84','4f5cd004-f011-11e5-91d1-61143f034f84','4f666934-f011-11e5-91d1-61143f034f84','4f70283e-f011-11e5-91d1-61143f034f84','4f7a85ea-f011-11e5-91d1-61143f034f84','4f8413d0-f011-11e5-91d1-61143f034f84','4f8d37c6-f011-11e5-91d1-61143f034f84','4f96000e-f011-11e5-91d1-61143f034f84','4fa0e604-f011-11e5-91d1-61143f034f84','4fac8cca-f011-11e5-91d1-61143f034f84') AND created_at IS NOT NULL
D, [2016-03-22T18:35:08.639395 #24939] DEBUG -- : (0.001794s) COMMIT
      returns a task

AFTER (v0.8.46):

PerfectQueue::Backend::RDBCompatBackend
  #acquire
    very large jobs
D, [2016-03-22T18:33:20.249724 #24776] DEBUG -- : (0.000090s) SET @@wait_timeout = 2147483
D, [2016-03-22T18:33:20.253954 #24776] DEBUG -- : (0.004107s) SET SQL_AUTO_IS_NULL=0
D, [2016-03-22T18:33:20.291939 #24776] DEBUG -- : (0.037849s) DELETE FROM `test_queues` WHERE timeout <= 458639200 AND created_at IS NULL
D, [2016-03-22T18:33:20.293357 #24776] DEBUG -- : (0.000102s) SET @@wait_timeout = 2147483
D, [2016-03-22T18:33:20.293484 #24776] DEBUG -- : (0.000077s) SET SQL_AUTO_IS_NULL=0
D, [2016-03-22T18:33:20.293660 #24776] DEBUG -- : (0.000112s) SELECT GET_LOCK('test_queues', 60) locked
D, [2016-03-22T18:33:20.655361 #24776] DEBUG -- : (0.361516s) SELECT id, timeout, data, created_at, resource
FROM `test_queues`
WHERE 1300000000 < timeout AND timeout <= 1458639200 AND timeout <= 1458639200
      AND created_at IS NOT NULL
ORDER BY timeout ASC
LIMIT 10

D, [2016-03-22T18:33:21.019104 #24776] DEBUG -- : (0.214267s) UPDATE `test_queues` SET timeout=1458639242 WHERE timeout <= 1458639200 AND id IN ('11c0c1b0-f011-11e5-91d1-61143f034f84','11cb220e-f011-11e5-91d1-61143f034f84','11d5c07e-f011-11e5-91d1-61143f034f84','11def70c-f011-11e5-91d1-61143f034f84','11e941b2-f011-11e5-91d1-61143f034f84','11f29708-f011-11e5-91d1-61143f034f84','11fb71c0-f011-11e5-91d1-61143f034f84','1204450c-f011-11e5-91d1-61143f034f84','120d6e16-f011-11e5-91d1-61143f034f84','1216833e-f011-11e5-91d1-61143f034f84') AND created_at IS NOT NULL
      returns a task

spec/rdb_stress.rb

@@ -25,6 +25,7 @@
     let (:task_token){ Backend::RDBCompatBackend::Token.new(key) }
     let (:alive_time){ 42 }
     let (:max_acquire){ 10 }
+
     context 'some tasks' do
       before do
         sql = nil
@@ -51,5 +52,28 @@
         expect(ary[0]).to be_an_instance_of(AcquiredTask)
       end
     end
+
+    context 'very large jobs' do
+      before do
+        sql = nil
+        sql = 'INSERT `test_queues` (id, timeout, data, created_at, resource) VALUES'
+        data = %<UNCOMPRESS(UNCOMPRESS(FROM_BASE64('6B8AAHic7c6xCYNQFEDRh8HGAawjmAmEOE7WsPziGoJ1xnAJLbJCVgiJbpBOkHOqW96IFN34nvvYpOvyuZXPIgAAAICTS6/hku1RfR9tffQNAAAA8Icxb+7r9AO74A1h')))>
+        200.times do |i|
+          t = now - 600 + i/1000
+          sql << "(UUID(),#{t},#{data},#{t},NULL),"
+        end
+        db.db.run sql.chop!
+        db.db.loggers << Logger.new($stderr)
+        db.db.sql_log_level = :debug
+      end
+      it 'returns a task' do
+        #db.instance_variable_set(:@cleanup_interval_count, 0)
+        #expect(db.db.instance_variable_get(:@default_dataset)).to receive(:delete).and_call_original
+        ary = db.acquire(alive_time, max_acquire, {})
+        expect(ary).to be_an_instance_of(Array)
+        expect(ary.size).to eq(10)
+        expect(ary[0]).to be_an_instance_of(AcquiredTask)
+      end
+    end
   end
 end