rgw: Remove pessimizing move of return values

rgw: Remove pessimizing move of return values

C++ performs copy elision of return values if:

1. The value is a local declared in the function (i.e. not a
   parameter)
2. The type of the value in the return statement is, modulo
   const-value qualification, identical to the return type of the
   function.

An RValue reference to a type is not identical, modulo const-value
qualification, to the type it is a reference to.

Do not std::move local values in the return statement unless the value
is a parameter or you will make the compiler perform a copy it would
iotherwise avoid.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>