Make SplitHeuristics.maxParallelSplits tunable

We continue to observe lock contention during the split in our environment, which is caused by mongos sending 5 split requests on the same chunk at the same time. All the 5 split requests failed due to lock contention, and mongos would try to split again and again. It makes the sharding become very slow.

I seems to me it's better to disable to parallel splits?

Attached is the detail logs on mongod.