Ibrahim Jarif 8f0752ad02 Add support for watching nil prefix in subscribe API (#1246) ...

This PR adds support for watching empty prefixes (all keys) in subscribe API.

To subscribe to all changes in badger, user can run
```go
db.Subscribe(ctx, handler, nil)
```
or
```go
db.Subscribe(ctx, handler, []byte{})
```

(cherry picked from commit 5b8218b9f8)

2020-03-13 21:48:56 +05:30

..

trie_test.go

Add support for watching nil prefix in subscribe API (#1246)

2020-03-13 21:48:56 +05:30

trie.go

Add support for watching nil prefix in subscribe API (#1246)

2020-03-13 21:48:56 +05:30