Split get_default() into two for deferred default construction and added forwarding...

Split get_default() into two for deferred default construction and added forwarding to speed up default element construction with rvalues

Summary:
As it stood currently folly::get_default() would unnecessarily
construct a value into the third parameter, which was unnecessary in the code
path where the element was found in the map.  Also the default value can be
forwarded to the return type in the code path where the element is not found
and an rvalue parameter is passed as the default value

Reviewed By: yfeldblum

Differential Revision: D6390315

fbshipit-source-id: ef692b827d5a36751b4eb1e12042869e8fbba2e5