Index can be made multikey if a prefix of it is a shard key (splitVectors fail)

The index used for a shard key cannot be a multikey.

Hence inserts into a sharded collection that would turn the relevant index into a multi-key index are prevented, but only if the index is identical with the shard key:

tried to insert object with no valid shard key for { a: 1.0, b: 1.0 } : { _id: ObjectId('518a5fe33c631895e210858a'), a: 1.0, b: [ 1.0, 2.0, 3.0 ] }

If the shard key is instead only a prefix of the index, we do not prevent such inserts for other fields. This causes the index to turn into a multikey index and splitVectors will fail.

Steps to reproduce

1. Set up sharded cluster
2. Connect to mongos, insert a document into a collection:

   > db.docs.insert({a:1, b:2, c:3, d:4})

3. Create index on all 4 fields:

   > db.docs.ensureIndex({a:1, b:1, c:1, d:1})

4. Enable sharding and shard collection on a prefix of the index:

   > sh.enableSharding('test')

   > sh.shardCollection('test.docs', {a:1, b:1})

5. Now insert a document that contains an array for c:

   > db.docs.insert({a:5, b:6, c:[1, 2, 3], d:7})

6. Switch to mongod and run a splitVector command

   > db.adminCommand({splitVector: 'test.docs', keyPattern: {a:1, b:1}, maxChunkSize: 1})

   {

   "errmsg" : "couldn't find index over splitting key { a: 1.0, b: 1.0 }",

   "ok" : 0

   }

Proposed behavior

Any insert turning the index into a multikey index should fail with an error, as it does when the index pattern is identical to the shard key.