db bench enhancements (SPDB-270)

[Yuval-Ariel]

db_bench: add SeekRandomWriteRandom test
combination of seek random and overwrite random.
all threads are doing seek and write interchangeably.

[Yuval-Ariel]

@erez-speedb , can you make sure the SeekRandomWriteRandom test works as expected?

[erez-speedb]

Build failed
$DEBUG_LEVEL is 0
CC tools/db_bench.o
CC tools/db_bench_tool.o
tools/db_bench_tool.cc: In member function 'void rocksdb::Benchmark::SeekRandomWriteRandom(rocksdb::ThreadState*)':
tools/db_bench_tool.cc:7070:26: error: 'struct rocksdb::WriteOptions' has no member named 'timestamp'
7070 | write_options_.timestamp = &ts;
| ^~~~~~~~~
make: *** [Makefile:2444: tools/db_bench_tool.o] Error 1

[Yuval-Ariel]

fixed

[erez-speedb]

Passed performnace tests