feat(Dgraph): Use Badger with new WAL format.

[jarifibrahim]

This PR moves Dgraph to the new version of Badger that includes the new WAL. Some changes were
needed to make this work in Dgraph.

• Make sure that cache sizes are not zero when needed in alpha, bulk loader, and live loader.
• Remove flags that are no longer needed since the loading modes were changed to only
  support mmap.
• Remove deprecated options when opening DBs.
• Update how cache metrics are reported.
• Various other changes required to make the build compile.

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This change is 

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[manishrjain]

Approved. Let's get it merged asap.

Reviewed 6 of 15 files at r1, 3 of 9 files at r2, 5 of 7 files at r3, 7 of 8 files at r5, 1 of 1 files at r6, 2 of 2 files at r7.
Reviewable status: 23 of 26 files reviewed, 3 unresolved discussions (waiting on @jarifibrahim and @manishrjain)

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go.sum, line 166 at r7 (raw file):

github.com/google/codesearch v1.0.0 h1:z4h5JoHkUS+GqxqPDrldC3Y0Qq0vHAGgaDEW5pWU/ys=
github.com/google/codesearch v1.0.0/go.mod h1:qCnXDFnak/trCmLaE50kgPte3AX9jSeruZexWEOivi0=
github.com/google/flatbuffers v1.12.0 h1:/PtAHvnBY4Kqnx/xCQ3OIV9uYcSFGScBsWI3Oogeh6w=

go mod tidy

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test.sh, line 244 at r7 (raw file):

if [[ :${TEST_SET}: == *:systest:* ]]; then
    # TODO: Fix this test. The fix consists of updating the test script to

This test isn't that useful. So, disabling it for now.

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dgraph/cmd/bulk/run.go, line 114 at r7 (raw file):

		"The compression level for Badger. A higher value uses more resources.")
	flag.Int64("badger.cache_mb", 64, "Total size of cache (in MB) per shard in reducer.")
	flag.String("badger.cache_percentage", "100,0",

maybe 70,30