Badger: Stop compaction after flushing the memtable. (dgraph-io#4761)

- Don't close the channell while closing the doWrite (dgraph-io#918)
- Close channel while closing the db
- Stop compaction after flushing the memtable
Signed-off-by: பாலாஜி ஜின்னா <balaji@dgraph.io>

(cherry picked from commit 2b39009229f09bc05bc229a2032e912b5c65d880)

Update CHANGELOG for v2.0.0-rc1. (dgraph-io#4783)

Dgraph v1.2.1

Dgraph v2.0.0-beta1

Update CHANGELOG for v1.2.0 release. (dgraph-io#4584)

Co-authored-by: Daniel Mai <daniel@dgraph.io>

Dgraph v1.2.0-rc1

Dgraph v1.1.1

Dgraph v1.0.18

Dgraph v1.1.1-rc2

Dgraph v1.1.1-rc1