TableInfo: Return valid Left key

[jarifibrahim]

TableInfo function returns table boundaries. This is used in dgraph to calculate tablet size. As seen in issue hypermodeinc/dgraph#5026, badger could return !badger!head or !badger!move keys as the start of the table. These are valid start keys for badger but they're invalid for dgraph.
This PR changes TableInfo function so that we return only valid table start. For a table which contains the following keys

!badger!Head!
!badger!Move!...
!badger!Move!...
a
b
za

The valid starting key (left key) should be a and the end key should be za.

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This change is 

[jarifibrahim]

There could be a table with only badgerPrefix keys. What should we do in that case? Skip the table?

[gja]

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)

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levels_test.go, line 580 at r1 (raw file):

		createAndOpen(db, kv, 1)

		// Without key count.

split this with t.Run() ?

[manishrjain]

Don't need this PR.

Reviewable status: 0 of 2 files reviewed, 2 unresolved discussions (waiting on @ashish-goswami, @jarifibrahim, and @manishrjain)