db_bench: Fix a RocksDB bug in the cleanup of a Benchmark when using multiple DBs (-num_multi_db)

[udi-speedb]

There is a bug in db_bench in the cleanup of the Benchmark (~Benchmark) when running db_bench with multiple db-s (-num_multi_db).
For example, when running any command such as:
`./db_bench -num_multi_db 2 -benchmarks fillseq -num 10000

The following occurs:
fillseq : 11.541 micros/op 86639 ops/sec; 9.6 MB/s **db_bench: db/column_family.cc:1494: rocksdb::ColumnFamilySet::~ColumnFamilySet(): Assertion last_ref failed.** Received signal 6 (Aborted)

#0   /lib/x86_64-linux-gnu/libc.so.6(gsignal+0xcb) [0x7f967e03e03b] ??	??:0	`
#1   /lib/x86_64-linux-gnu/libc.so.6(abort+0x12b) [0x7f967e01d859] ??	??:0	
#2   /lib/x86_64-linux-gnu/libc.so.6(+0x22729) [0x7f967e01d729] ??	??:0	
#3   /lib/x86_64-linux-gnu/libc.so.6(+0x34006) [0x7f967e02f006] ??	??:0	
#4   ./db_bench(+0x65b748) [0x557921753748] ??	??:0	
#5   ./db_bench(+0x319e10) [0x557921411e10] ??	??:0	
#6   ./db_bench(+0x310981) [0x557921408981] ??	??:0	
#7   ./db_bench(+0x2ef7ac) [0x5579213e77ac] ??	??:0	
#8   ./db_bench(+0x2ef9dc) [0x5579213e79dc] ??	??:0	
#9   ./db_bench(+0x159876) [0x557921251876] ??	??:0	
#10  ./db_bench(+0x14ce45) [0x557921244e45] ??	??:0	
#11  ./db_bench(+0x1199b9) [0x5579212119b9] ??	??:0	
#12  ./db_bench(+0x119f71) [0x557921211f71] ??	??:0	
#13  ./db_bench(+0x11a3da) [0x5579212123da] ??	??:0	
#14  ./db_bench(+0x7e373) [0x557921176373] ??	??:0	
#15  ./db_bench(+0x7e3b4) [0x5579211763b4] ??	??:0	
#16  ./db_bench(+0xbefe6) [0x5579211b6fe6] ??	??:0	
#17  ./db_bench(+0xaf8ef) [0x5579211a78ef] ??	??:0	
#18  ./db_bench(+0x65b42) [0x55792115db42] ??	??:0	
#19  ./db_bench(+0x6626d) [0x55792115e26d] ??	??:0	
#20  ./db_bench(+0x6442d) [0x55792115c42d] ??	??:0	
#21  /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3) [0x7f967e01f0b3] ??	??:0	
#22  ./db_bench(+0x6434e) [0x55792115c34e] ??	??:0	
Aborted

[udi-speedb]

I see the bug in the latest version of db_bench_tool.cc in RocksDB's GitHub:
void DeleteDBs() { db_.DeleteDBs(); for (const DBWithColumnFamilies& dbwcf : multi_dbs_) { delete dbwcf.db; } }