node, async-wrap: remove MakeDomainCallback

C++ won't deoptimize like JS if specific conditional branches are
sporadically met in the future. Combined with the amount of code
duplication removal and simplified maintenance complexity, it makes more
sense to merge MakeCallback and MakeDomainCallback.

Additionally, type casting in V8 before verifying what that type is will
cause V8 to abort in debug mode if that type isn't what was expected.
Fix this by first checking the v8::Value before casting.

PR-URL: https://github.com/joyent/node/pull/8110


Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>
Reviewed-by: Alexis Campailla <alexis@janeasystems.com>
Reviewed-by: Julien Gilli <julien.gilli@joyent.com>