Enable background flush thread by default and fix issues related to it

Summary:
Enable background flush thread in this patch and fix unit tests with:
(1) After background flush, schedule a background compaction if condition satisfied;
(2) Fix a bug that if universal compaction is enabled and number of levels are set to be 0, compaction will not be automatically triggered
(3) Fix unit tests to wait for compaction to finish instead of flush, before checking the compaction results.

Test Plan: pass all unit tests

Reviewers: haobo, xjin, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13461

db/db_test.cc

@@ -1850,6 +1850,8 @@ TEST(DBTest, UniversalCompactionSizeAmplification) {
   // but will instead trigger size amplification.
   dbfull()->Flush(FlushOptions());
 
+  dbfull()->TEST_WaitForCompact();
+
   // Verify that size amplification did occur
   ASSERT_EQ(NumTableFilesAtLevel(0), 1);
 }