src: add UsingDomain check

Node stores when domains are in use, so might as well add the ability to
quickly query this value.
req_wrap: only check for domain if in use

Use UsingDomain to check if domain object needs to be retrieved.
src: get rid of compiler warning

Removed the following compiler warning from clang:

warning: adding 'int' to a string does not append to the string
[-Wstring-plus-int]