Use 'batch get' to improve the performance of 'multi...exec'

[shafreeck]

It is inefficient to fetch values from TiKV multiple times sequentially. For example

multi
hset uid name tom
hset uid age 23
hset uid career engineer
exec

The fields of the uid should be checked its existence before actually inserting or updating a field. Fetching every field (name, age, career) sequentially causes high latency. Maybe we can fetch all the fields at once by batch.