add GetAllKeys

[roseduan]

A new feature: list all stored keys conatain all datastructure types.

Here is my idea:

• add GetKeys command in different data types respectively(GetStrsKeys, GetListKeys, GetHashKeys...)
• add GetAllKeys command in db.go, retrieve all data types GetKeys concurrently

The GetAllKeys method like this:
func (db *RoseDB) GetAllKeys(dataTypes... DataType) (map[DataType][][]byte, error)

[roseduan]

I have add a GetStrsKeys method in strs.go

[Reid00]

I have added pr #190