opt(txn commits): Optimize txns by passing Skiplists to Badger

[manishrjain]

In this PR, we use concurrent mutations system, introduced in #7694 , to generate Skiplists directly from a transaction, even before it is committed.

When a transaction does get committed, we replace the key timestamps with the commit timestamp, in-place within the Skiplist. And in case of multiple transactions being committed, merge them together into one bigger Skiplist during the serial commit.

This Skiplist is then handed over to Badger, bypassing its value log and WAL system. Instead, when Badger persists the skiplist into L0 tables, Dgraph gets a callback, which it uses to decide when it is safe to take a Raft snapshot / checkpoint. So, we no longer need Badger's WAL.

Furthermore, live loader is also optimized to better deal with the conflicting transactions, and retrying them as frequently as possible, instead of only towards the end. The updated output from live loader shows we're able to deal with stragglers much better, with them causing almost no delays with this PR.

There's also a bug fix in this PR. The Raft checkpointing code was not getting run as often as it should. This PR fixes that.

Master:
[18:48:34-0700] Elapsed: 08m45s Txns: 21222 N-Quads: 21,221,870 N-Quads/s: 9,033 Aborts: 0
Number of TXs run            : 21240
Number of N-Quads processed  : 21239870
Time spent                   : 8m49.039284385s
N-Quads processed per second : 40150

This PR:
Number of TXs run            : 21240
Number of N-Quads processed  : 21239870
Time spent                   : 6m56.641399434s
N-Quads processed per second : 51057

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This change is 

[joshua-goldstein]

For reference the relevant PR in Badger is dgraph-io/badger#1696