Prevent slowdowns and stalls in PreShutdown tests

Summary:
The preshutdown tests check for stopped compactions/flushes.
Removing stalls on the write path.

Test Plan: DBTests.PreShutdown*

Reviewers: yhchiang, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D35037

db/db_test.cc

@@ -10283,6 +10283,8 @@ TEST(DBTest, PreShutdownMultipleCompaction) {
   options.level0_file_num_compaction_trigger = kNumL0Files;
   options.max_bytes_for_level_multiplier = 2;
   options.max_background_compactions = kLowPriCount;
+  options.level0_stop_writes_trigger = 1 << 10;
+  options.level0_slowdown_writes_trigger = 1 << 10;
 
   TryReopen(options);
   Random rnd(301);
@@ -10371,6 +10373,8 @@ TEST(DBTest, PreShutdownCompactionMiddle) {
   options.level0_file_num_compaction_trigger = kNumL0Files;
   options.max_bytes_for_level_multiplier = 2;
   options.max_background_compactions = kLowPriCount;
+  options.level0_stop_writes_trigger = 1 << 10;
+  options.level0_slowdown_writes_trigger = 1 << 10;
 
   TryReopen(options);
   Random rnd(301);