common/config_opts.h: use string for rocksdb options.

common/config_opts.h: use string for rocksdb options.

Now we can drop the ugly rocksdb interpret options, and
have
  // rocksdb options that will be used for keyvaluestore(if backend is rocksdb)
  OPTION(keyvaluestore_rocksdb_options, OPT_STR, "")
  // rocksdb options that will be used for omap(if omap_backend is rocksdb)
  OPTION(omap_rocksdb_options, OPT_STR, "")
  // rocksdb options that will be used in monstore
  OPTION(mon_rocksdb_options, OPT_STR, "")

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>