fix(ludicrous_mode): fixing type inversion in ludicrous mode

[aman-bansal]

This PR aims to solve the crashes we have seen in ludicrous mode due to type inversion error. One such crash is

I0315 18:37:10.987252      18 log.go:34] 2 [logterm: 45, index: 1734160, vote: 2] rejected MsgPreVote from 1 [logterm: 44, index: 1734158] at term 45
panic: interface conversion: interface {} is string, not time.Time
goroutine 3766 [running]:
github.com/dgraph-io/dgraph/tok.DayTokenizer.Tokens(0x1b661e0, 0xc01646c470, 0x1e, 0x1e, 0xdb1ac1f11fa1c8e2, 0x7f1300000000, 0x1b661e0)
	/ext-go/1/src/github.com/dgraph-io/dgraph/tok/tok.go:249 +0x2f8
github.com/dgraph-io/dgraph/tok.BuildTokens(0x1b661e0, 0xc01646c470, 0x1fb6c20, 0x2e2ff78, 0x1fb6c20, 0x2e2ff78, 0xc01646c470, 0x0, 0x0)
	/ext-go/1/src/github.com/dgraph-io/dgraph/tok/tok.go:108 +0x63
github.com/dgraph-io/dgraph/worker.generateTokenKeys(0xc00e546c00, 0xc01646c450, 0x1, 0x1, 0x12, 0xc01646c400, 0x1, 0x1, 0xc00ebf6e00)
	/ext-go/1/src/github.com/dgraph-io/dgraph/worker/executor.go:83 +0x237
github.com/dgraph-io/dgraph/worker.generateConflictKeys(0x1faf2e0, 0xc000046070, 0xc0a093a300, 0x0)
	/ext-go/1/src/github.com/dgraph-io/dgraph/worker/executor.go:120 +0x47d
github.com/dgraph-io/dgraph/worker.(*executor).processMutationCh(0xc00e3bf220, 0x1faf2e0, 0xc000046070, 0xc02ae403c0)
	/ext-go/1/src/github.com/dgraph-io/dgraph/worker/executor.go:220 +0x46d
created by github.com/dgraph-io/dgraph/worker.(*executor).getChannel
	/ext-go/1/src/github.com/dgraph-io/dgraph/worker/executor.go:279 +0x130
[Sentry] 2021/03/15 18:37:12 Sending fatal event [215c647f319d41d2aa93a0e0d4d01b84] to o318308.ingest.sentry.io project: 5208688
[Sentry] 2021/03/15 18:37:13 Buffer flushed successfully.

The catch here is that, if there is no way to type cast values, do create tokens, skip it.

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This change is 

[ahsanbarkati]

Looks good. We should write some unit tests for these functions.

[aman-bansal]

This fixes the crashes we are seeing with the tokenizers. We have planned major refactoring around ludicrous mode. would be better to write tests then :).