Improvements/Refactoring

[pawanrawal]

1. Call pl.Length() only if attr has @count index.
2. Fix inconsistent semantics with uid function. Earlier uid function at root didn't have sg.Attr as empty but uid function in children would have had sg.Attr as uid. Now sg.Attr is empty for uid filter Subgraph.

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This change is 

[ashwin95r]

Also, Empty uid()

{
  me(func: uid())
}

throws:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xb71c37]

goroutine 9486 [running]:
github.com/dgraph-io/dgraph/query.ProcessGraph(0x12f9e20, 0xc42c2a0ba0, 0xc42020d8c0, 0x0, 0xc42c2a0c00)
	/home/ashwin/go/src/github.com/dgraph-io/dgraph/query/query.go:1594 +0x77
created by github.com/dgraph-io/dgraph/query.(*QueryRequest).ProcessQuery
	/home/ashwin/go/src/github.com/dgraph-io/dgraph/query/query.go:2204 +0x4a9

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Review status: 0 of 4 files reviewed at latest revision, 1 unresolved discussion.

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posting/index.go, line 188 at r1 (raw file):

	x.PredicateStats.Add(fmt.Sprintf("r.%s", edge.Attr), 1)

	if countAfter != countBefore && hasCountIndex {

hasCountIndex && countAfter != countBefore

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Comments from Reviewable

[ashwin95r]

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Review status: 0 of 7 files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[tzdybal]

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Reviewed 3 of 4 files at r1, 4 of 4 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[manishrjain]

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Reviewed 3 of 4 files at r1, 4 of 4 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion, some commit checks broke.

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Comments from Reviewable