Use system-wide LevelDB instead of bundled one

src/CMakeLists.txt

src/leveldb/.gitignore

src/leveldb/AUTHORS

src/leveldb/CMakeLists.txt

src/leveldb/LICENSE

src/leveldb/NEWS

src/leveldb/README

src/leveldb/TODO

src/leveldb/build_detect_platform

src/leveldb/db/builder.cc

src/leveldb/db/builder.h

src/leveldb/db/c.cc

src/leveldb/db/c_test.c

src/leveldb/db/corruption_test.cc

src/leveldb/db/db_bench.cc

src/leveldb/db/db_impl.cc

src/leveldb/db/db_impl.h

src/leveldb/db/db_iter.cc

src/leveldb/db/db_iter.h

src/leveldb/db/db_test.cc