Ibrahim Jarif 6b890a6450 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 c6c1e5ec76)

2020-03-24 15:46:15 +05:30

..

trie_test.go

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

2020-03-24 15:46:15 +05:30

trie.go

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

2020-03-24 15:46:15 +05:30