db_bench support user defined timestamp

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### Expected behavior
db_bench support enabling user defined timestamp

### Actual behavior
some errors encountered in while benchmarking this feature with the tool

### Steps to reproduce the behavior
> Note: Please use Issues only for bug reports. For questions, discussions, feature requests, etc. post to dev group: https://groups.google.com/forum/#!forum/rocksdb or https://www.facebook.com/groups/rocksdb.dev
./db_bench --num=10000000 --disable_wal --value_size=16 --report_interval_seconds=1 --db=/tmp/timestamp --statistics --benchmarks=fillrandom --delayed_write_rate=$((128*1024*1024)) --max_background_flushes=4 --max_background_compactions=8 --threads=16 -user_timestamp_size=8 --compression_type=none

./db_bench --num=10000000 --disable_wal --value_size=16 --report_interval_seconds=1 --db=/tmp/timestamp --statistics --benchmarks=readrandom --delayed_write_rate=$((128*1024*1024)) --cache_size=$(( 4 *1024 * 1024)) --max_write_buffer_number=6 --max_background_flushes=4 --max_background_compactions=8 -user_timestamp_size=8 --threads=1 --compression_type=none --use_existing_db --duration=60 --read_with_latest_user_timestamp

On the write path: the keys that are created are without a timestamp. The command batch.clear() also reset the value of default_cf_ts_sz_ and the db_bench does not set it back
On the read path: only a small fraction of the keys, this is suspected to be an issue with the tool instead of the feature's own bug