fix(Dgraph): check for deleteBelowTs in pIterator.valid

[ajeetdsouza]

Fixes DGRAPH-2705.

To reproduce, first insert a large number of edges of the form:

<0x1> <follows> <0x2> .
<0x1> <follows> <0x3> .
<0x1> <follows> <0x4> .
...

and then delete all of them:

<0x1> <follows> * .

After this, we can still see a few edges left despite the delete succeeding.

The reason this happens is that each part in a pIterator is initialized with a valid function, which checks if the iterator is currently on a valid block of UIDs, and skips to the next valid block if not. In contrast, the next function advances to the next valid block regardless of whether the current block is valid or not.

The next function checks the deleteBelowTs to ensure that we don't end up reading deleted data from the disk, however, the valid function did not earlier include that check. As a result, we would end up reading one deleted UID from each part in the split posting list, after which we would call next, which would discard the rest of the part. This PR adds the same deleteBelowTs check to the valid function as well, to ensure that this does not happen.

This change is 

[pawanrawal]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @martinmr)

[pawanrawal]

Can you add a test for this?

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @martinmr)

[jarifibrahim]

This PR needs a better description.

[manishrjain]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @martinmr and @vvbalaji-dgraph)