new cache double check

[Kimmy Kuang]

what are the advantages doing double check table name exists in cache2go.Cache?

[Christian Muehlhaeuser]

See: #11

Essentially: between read-locking and locking, the table might have been created in another thread.

[Kimmy Kuang]

thanks.

[Deep-Coder-zhui]

@muesli ，I think the RLock() may be not needed because of the double check