fix(Dgraph): Don't store start_ts in postings.

[martinmr]

Clear out the start_ts field in the postings of deltas before they are
marshalled to avoid storing that field in disk. This field is only meant
to be used during in-memory processing.

Related to https://discuss.dgraph.io/t/start-ts-not-being-cleared-before-postings-are-written-to-disk/9146

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This change is 

[martinmr]

This is what the debug tool is showing after and before the change.

After change.
➜  ~ dgraph debug -p=p -o=false -l 000006667269656e64000000000000000001 -y
[Decoder]: Using assembly version of decoder
I0803 15:15:18.477179 1280891 util_ee.go:126] KeyReader instantiated of type <nil>
Opening DB: p
badger 2020/08/03 15:15:18 INFO: All 1 tables opened in 0s
badger 2020/08/03 15:15:18 INFO: Replaying file id: 0 at offset: 593
badger 2020/08/03 15:15:18 INFO: Replay took: 1.72µs
==> key: 000006667269656e64000000000000000001. PK: {ByteType:0 Attr:friend Uid:1 HasStartUid:false StartUid:0 Term: Count:0 bytePrefix:0}
ts: 10 {item}{delta}
 postings: [uid:2 val_type:UID op:1  uid:3 val_type:UID op:1 ]  Uid: 2 Op: 1
 Uid: 3 Op: 1


badger 2020/08/03 15:15:18 INFO: Got compaction priority: {level:0 score:1.73 dropPrefixes:[]}

Before change
➜  ~ dgraph debug -p=p -o=false -l 000006667269656e64000000000000000001 -y
[Decoder]: Using assembly version of decoder
I0803 15:16:12.027965 1282216 util_ee.go:126] KeyReader instantiated of type <nil>
Opening DB: p
badger 2020/08/03 15:16:12 INFO: All 1 tables opened in 0s
badger 2020/08/03 15:16:12 INFO: Replaying file id: 0 at offset: 366
badger 2020/08/03 15:16:12 INFO: Replay took: 13.384µs
==> key: 000006667269656e64000000000000000001. PK: {ByteType:0 Attr:friend Uid:1 HasStartUid:false StartUid:0 Term: Count:0 bytePrefix:0}
ts: 4 {item}{delta}
 postings: [uid:2 val_type:UID op:1 start_ts:2  uid:3 val_type:UID op:1 start_ts:2 ]  Uid: 2 Op: 1
 Uid: 3 Op: 1


badger 2020/08/03 15:16:12 INFO: Got compaction priority: {level:0 score:1.73 dropPrefixes:[]}
badger 2020/08/03 15:16:12 INFO: Running for level: 0
badger 2020/08/03 15:16:12 INFO: LOG Compact 0->1, del 2 tables, add 1 tables, took 2.645601ms
badger 2020/08/03 15:16:12 INFO: Compaction for level: 0 DONE
badger 2020/08/03 15:16:12 INFO: Force compaction on level 0 done

[parasssh]

Ok. But doesn't start_ts aid in trouble-shooting?

[martinmr]

Once it's on disk, the start ts is the same as the version of the key so it's just duplicate information. There are comments on the code saying start ts is just meant for in-memory processing but I guess at some point the code and the comments went out of sync.

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)