kv: make delete range optional on number of keys

kv: make delete range optional on number of keys

Actually, we may only wanna let the objects with real big number of omap to
use delete range. if not, we find too many tombstones will have side-effects
on performance of kv.

refer: https://github.com/facebook/rocksdb/wiki/DeleteRange-Implementation

Signed-off-by: Zengran Zhang <zhangzengran@sangfor.com.cn>