domain: change name for domain setup

The name UsingDomains is misleading for a function that initializes
domains for use.
src: add using_domains to node_internals

Make it easy to check if domains are in use
req_wrap: only grab domain object if in use
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]