[tools] Add a tool to stress test concurrent writing to levelDB

Summary:
Created a tool that runs multiple threads that concurrently read and write to levelDB.
All writes to the DB are stored in an in-memory hashtable and verified at the end of the
test. All writes for a given key are serialzied.

Test Plan:
 - Verified by writing only a few keys and logging all writes and verifying that values read and written are correct.
 - Verified correctness of value generator.
 - Ran with various parameters of number of keys, locks, and threads.

Reviewers: dhruba, MarkCallaghan, heyongqiang

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D5829

Makefile

@@ -57,6 +57,7 @@ TESTS = \
 TOOLS = \
 	manifest_dump \
         sst_dump \
+        db_stress \
         ldb
 
 PROGRAMS = db_bench $(TESTS) $(TOOLS)
@@ -112,6 +113,9 @@ $(LIBRARY): $(LIBOBJECTS)
 db_bench: db/db_bench.o $(LIBOBJECTS) $(TESTUTIL)
 	$(CXX) db/db_bench.o $(LIBOBJECTS) $(TESTUTIL) $(EXEC_LDFLAGS) -o $@  $(LDFLAGS)
 
+db_stress: tools/db_stress.o $(LIBOBJECTS) $(TESTUTIL)
+	$(CXX) tools/db_stress.o $(LIBOBJECTS) $(TESTUTIL) $(EXEC_LDFLAGS) -o $@  $(LDFLAGS)
+
 db_bench_sqlite3: doc/bench/db_bench_sqlite3.o $(LIBOBJECTS) $(TESTUTIL)
 	$(CXX) doc/bench/db_bench_sqlite3.o $(LIBOBJECTS) $(TESTUTIL) -o $@ $(LDFLAGS) -lsqlite3