fix(badger): Update badger to v20.07.0-rc1

[jarifibrahim]

Fixes #5573

This commit brings the following new changes from badger.

This description was generated using this script:

#!/bin/bash

set -e
GHORG=${GHORG:-dgraph-io}
GHREPO=${GHREPO:-badger}
cat <<EOF
This description was generated using this script:
\`\`\`sh
`cat $0`
\`\`\`
Invoked as:

    `echo GHORG=${GHORG} GHREPO=${GHREPO} $(basename $0) ${@:1}`

EOF
git log --oneline --reverse ${@:1} \
  | sed -E "s/^(\S{7}\s)//g" \
  | sed -E "s/([\s|\(| ])#([0-9]+)/\1${GHORG}\/${GHREPO}#\2/g" \

Invoked as:

GHORG=dgraph-io GHREPO=badger changes.sh 14386ac9b76...0dfb8b45d4a4

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

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This change is 

[martinmr]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)