Rename GetMRSplits to SplitQuery

The functionality is not specific to MapReduce, any job that wants to
split a large query into non-overlapping sub queries can use this end point.
SplitQuery should be shard agnostic

Specify total number of splits instead of splitsPerShard
Rename QuerySplit to SplitQueryPart
Merge pull request #144 from youtube/split-query

Minor refactor of GetMRSplits