Improve the BatchGet behavior for TiKV

[shafreeck]

Set the iterator bound when using BatchGet.

Think about this case, we have two keys: a, z and there are 1 million keys are deleted between a and z. When we look up the key b, RocksDB internal will scan over all the 1 million invisible keys and return the next visible key which is z. It is a big overhead here. RocksDB supports to set the iterator bound like c which will stop scanning when meeting the bound.

In the BatchGet case, Ex. mget b e, It is not necessary to return z when seeks b.

About this issue, you should do these following things:

• Verify RocksDB that behave like what described above.
• Figure out how the BatchGet in TiKV implemented, and why to use an iterator to look up a key.
• Improve the TiKV implementation and the Titan code.