Update badger to commit 91c31ebe8c22

[jarifibrahim]

Important changes

 - Changes to overlap check in compaction.
 - Remove 'this entry should've been caught' log.
 - Changes to write stalling on level 0 and 1.
 - Compression is disabled by default in Badger.
 - Bloom filter caching in a separate ristretto cache.
 - Compression/Encryption in background.
 - Disable cache by default in badger.

The following new changes are being added
git log ab4352b00a17...91c31ebe8c22

91c31eb Disable cache by default (#1257)
eaf64c0 Add separate cache for bloom filters (#1260)
1bcbefc Add BypassDirLock option (#1243)
c6c1e5e Add support for watching nil prefix in subscribe API (#1246)
b13b927 Compress/Encrypt Blocks in the background (#1227)
bdb2b13 fix changelog for v2.0.2 (#1244)
8dbc982 Add Dkron to README (#1241)
3d95b94 Remove coveralls from Travis Build(#1219)
5b4c0a6 Fix ValueThreshold for in-memory mode (#1235)
617ed7c Initialize vlog before starting compactions in db.Open (#1226)
e908818 Update CHANGELOG for Badger 2.0.2 release. (#1230)
bce069c Fix int overflow for 32bit (#1216)
e029e93 Remove ExampleDB_Subscribe Test (#1214)
8734e3a Add missing package to README for badger.NewEntry (#1223)
78d405a Replace t.Fatal with require.NoError in tests (#1213)
c51748e Fix flaky TestPageBufferReader2 test (#1210)
eee1602 Change else-if statements to idiomatic switch statements. (#1207)
3e25d77 Rework concurrency semantics of valueLog.maxFid (#1184) (#1187)
4676ca9 Add support for caching bloomfilters (#1204)
c3333a5 Disable compression and set ZSTD Compression Level to 1 (#1191)
0acb3f6 Fix L0/L1 stall test (#1201)
7e5a956 Support disabling the cache completely. (#1183) (#1185)
82381ac Update ristretto to version  8f368f2 (#1195)
3747be5 Improve write stalling on level 0 and 1
5870b7b Run all tests on CI (#1189)
01a00cb Add Jaegar to list of projects (#1192)
9d6512b Use fastRand instead of locked-rand in skiplist (#1173)
2698bfc Avoid sync in inmemory mode (#1190)
2a90c66 Remove the 'this entry should've caught' log from value.go (#1170)
0a06173 Fix checkOverlap in compaction (#1166)
0f2e629 Fix windows build (#1177)
03af216 Fix commit sha for WithInMemory in CHANGELOG. (#1172)
23a73cd Update CHANGELOG for v2.0.1 release. (#1181)
465f28a Cast sz to uint32 to fix compilation on 32 bit (#1175)
ea01d38 Rename option builder from WithInmemory to WithInMemory. (#1169)
df99253 Remove ErrGCInMemoryMode in CHANGELOG. (#1171)
8dfdd6d Adding changes for 2.0.1 so far (#1168)

This change is 

[martinmr]

There was a few tests failing. I reran the CI in case those are just flaky failures.

once all the tests are passing.

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