Summary: Add a benchmark to `db_bench`. In this benchmark, a write thread will populate time series data in the format of 'id | timestamp', and multiple read threads will randomly retrieve all data from one id at a time.

Test Plan: Run the benchmark: `num=134217728;bpl=536870912;mb=67108864;overlap=10;mcz=2;del=300000000;levels=6;ctrig=4;delay=8;stop=12;wbn=3;mbc=20;wbs=134217728;dds=0;sync=0;t=32;vs=800;bs=4096;cs=17179869184;of=500000;wps=0;si=10000000; kir=100000; dir=/data/users/jhli/test/; ./db_bench --benchmarks=timeseries --disable_seek_compaction=1 --mmap_read=0 --statistics=1 --histogram=1 --num=$num --threads=$t --value_size=$vs --block_size=$bs --cache_size=$cs --bloom_bits=10 --cache_numshardbits=6 --open_files=$of --verify_checksum=1 --db=$dir --sync=$sync --disable_wal=0 --compression_type=none --stats_interval=$si --compression_ratio=1 --disable_data_sync=$dds --write_buffer_size=$wbs --target_file_size_base=$mb --max_write_buffer_number=$wbn --max_background_compactions=$mbc --level0_file_num_compaction_trigger=$ctrig --level0_slowdown_writes_trigger=$delay --level0_stop_writes_trigger=$stop --num_levels=$levels --delete_obsolete_files_period_micros=$del --min_level_to_compress=$mcz --max_grandparent_overlap_factor=$overlap --stats_per_interval=1 --max_bytes_for_level_base=$bpl --use_existing_db=0 --key_id_range=$kir`

Reviewers: andrewkr, sdong

Reviewed By: sdong

Subscribers: lgalanis, andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D60651