Prevent concurrent multiple opens of leveldb database.

Summary:
The fcntl call cannot detect lock conflicts when invoked multiple times
from the same thread.
Use a static lockedFile Set to record the paths that are locked.
A lockfile request checks to see if htis filename already exists in
lockedFiles, if so, then it triggers an error. Otherwise, it inserts
the filename in the lockedFiles Set.
A unlock file request verifies that the filename is in the lockedFiles
set and removes it from lockedFiles set.

Test Plan: unit test attached

Reviewers: heyongqiang

Reviewed By: heyongqiang

Differential Revision: https://reviews.facebook.net/D4755

Makefile

@@ -56,7 +56,6 @@ TESTS = \
 	write_batch_test \
 	filelock_test
 
-
 TOOLS = \
 	manifest_dump \
 	leveldb_shell \
@@ -198,6 +197,10 @@ sst_dump: tools/sst_dump.o $(LIBOBJECTS)
 $(VERSIONFILE):	build_detect_version
 	$(shell ./build_detect_platform build_config.mk)
 
+filelock_test: util/filelock_test.o $(LIBOBJECTS) $(TESTHARNESS)
+	$(CXX) util/filelock_test.o $(LIBOBJECTS) $(TESTHARNESS) -o $@ $(LDFLAGS)
+
+
 ifeq ($(PLATFORM), IOS)
 # For iOS, create universal object files to be used on both the simulator and
 # a device.