Reject requests with predicates larger than the max size allowed.

[martinmr]

This change introduces checks in server.go to verify that Alter,
Mutation, and Query requests that includes predicates larger than 2^16
characters are rejected before being processed.

Fixes #3049

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This change is 

[manishrjain]

One comment to consolidate the length checking of the predicate into a single function returning an error, which gets called from all the other places.

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @martinmr)

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edgraph/server.go, line 360 at r1 (raw file):

		if len(update.Predicate) > math.MaxUint16 {
			return nil, fmt.Errorf("Predicate size cannot be bigger than 2^16")

Predicate name length cannot be

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edgraph/server.go, line 782 at r1 (raw file):

	for _, nq := range set {
		if len(nq.Predicate) > math.MaxUint16 {
			return x.Errorf("Predicate size cannot be bigger than 2^16")

name length. Also, mention the predicate which has that length, maybe the first 80 bytes or something.

x.Errorf("Predicate name length cannot be bigger than 2^16. Predicate: %s...", nq.Predicate[:80])

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edgraph/server.go, line 797 at r1 (raw file):

	for _, nq := range del {
		if len(nq.Predicate) > math.MaxUint16 {
			return x.Errorf("Predicate size cannot be bigger than 2^16")

name length.

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edgraph/server.go, line 845 at r1 (raw file):

	for _, q := range queries {
		if len(q.Attr) > math.MaxUint16 {
			return fmt.Errorf("Predicate size cannot be bigger than 2^16")

Mention the predicate.

I see this error in multiple places, can you consolidate it all by creating a function to 'validatePred(...) error', so if it returns a non-nil, you can just return it directly.