Slightly modify kvdk_engine_unsorted to improve performance, also fix some compiler warning.

[ZiyanShi]

What's changed

1. Set C++ standard to C++17 in CMakeLists.txt to allow unittest.cpp to include <filesystem> header.
2. Change default parameters of KVDK engine.
   2.1. pmem_segment_blocks determines the maximum size of a key-value pair. Setting the parameter to 1024 allows the engine to store key-value pair of maximum size of approximately 64KB.
   2.2. hash_bucket_num is a critical parameter for performance tuning. KVDK engine currently does not support rehashing. With the load factor increasing, time for looking up a key will increase linearly accordingly. Load factor can be estimated by kv-pairs/(hash_bucket_num*7). Load factor 0.7 is suggested for best performance and DRAM utilization. In other words, it's recommended to set hash_bucket_num to kv-pairs/5. A larger parameter will not improve performance much, but will cost more DRAM.
3. Remove unused local variable, as the compiler will complain.