Add interfaces for segment key bounds, key bounds will be used to speed up point lookup on the primary key index of each segment. For the detail, see DSIP-018:https://cwiki.apache.org/confluence/display/DORIS/DSIP-018%3A+Support+Merge-On-Write+implementation+for+UNIQUE+KEY+data+model KeyBounds will be updated by BetaRowsetWriter, will be used to construct a RowsetTree(based on IntervalTree, will be added through next patch)