refactor(conf): use DSN_DEFINE_int64 to load int64 type of configs

[acelyc111]

#1323

This patch refactors the code to use DSN_DEFINE_int64 instead of dsn_config_get_value_int64/uint64 to load int64 type of configurations, and doesn't introduce any functional changes.

• all default value and most of description are kept as before
• move the defination of flags closer to the places where they're used
• remove checkpoint_min_decree_gap which is never used

NOTE: rocksdb_max_open_files is type of int32 because _db_opts.max_open_files is type of int32.