This commit brings following new changes from badger
This commit also disable conflict detection in badger to save memory.

```
Fix assert in background compression and encryption. (dgraph-io/badger#1366)
Avoid panic in filltables() (dgraph-io/badger#1365)
Force KeepL0InMemory to be true when InMemory is true (dgraph-io/badger#1375)
Tests: Use t.Parallel in TestIteratePrefix tests (dgraph-io/badger#1377)
Remove second initialization of writech in Open (dgraph-io/badger#1382)
Increase default valueThreshold from 32B to 1KB (dgraph-io/badger#1346)
pre allocate cache key for the block cache and the bloom filter cache (dgraph-io/badger#1371)
Rework DB.DropPrefix (dgraph-io/badger#1381)
Update head while replaying value log (dgraph-io/badger#1372)
Update ristretto to commit f66de99 (dgraph-io/badger#1391)
Enable cross-compiled 32bit tests on TravisCI (dgraph-io/badger#1392)
Avoid panic on multiple closer.Signal calls (dgraph-io/badger#1401)
Add a contribution guide (dgraph-io/badger#1379)
add assert to check integer overflow for table size (dgraph-io/badger#1402)
return error if the vlog writes exceeds more that 4GB. (dgraph-io/badger#1400)
Revert "add assert to check integer overflow for table size (dgraph-io/badger#1402)" (dgraph-io/badger#1406)
Revert "fix: Fix race condition in block.incRef (dgraph-io/badger#1337)" (dgraph-io/badger#1407)
Revert "Buffer pool for decompression (dgraph-io/badger#1308)" (dgraph-io/badger#1408)
Revert "Compress/Encrypt Blocks in the background (dgraph-io/badger#1227)" (dgraph-io/badger#1409)
Add missing changelog for v2.0.3 (dgraph-io/badger#1410)
Changelog for v20.07.0 (dgraph-io/badger#1411)
```