[bind10-dev] a couple of small things about ACL loader

[JINMEI Tatuya / 神明達哉]

While working on #1069, I came up with a couple of comments/suggestions
about the ACL loader.

1. why is CheckCreator::create() not a const member function?
   Existing examples don't seem to need to modify the class, and
   considering its nature (essentially it works as a factory), I
   suspect it can naturally be a const member function.

2. I'm afraid it's too restrictive to specify the default action
   through the constructor of Loader, especially if the expected usage
   is to use per-context singleton loader.  For example, both b10-auth
   and b10-resolver would use the RequestContext, but the reasonable
   default action would (or might) be different: for auth, "default
   accept" probably makes most sense, while "default reject" would be
   most reasonable for the resolver.  But we cannot customize the
   default as long as we use the singleton loader (and not using it
   would be inconvenient because the application would have to
   register default creators).  Instead, I'd suggest we specify the
   default action via the load() method (and if it's deemed to be
   reasonable, we can specify the default value).

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JINMEI, Tatuya