Support fragment references using $anchor

[Whathecode]

As is, this PR already resolves the first test case in V201909JsonSchemaTest.testAnchorValidator(), but not the latter two which include absolute URIs.

It causes java.io.FileNotFoundException: http://localhost:1234/bar due to "$ref": "http://localhost:1234/bar#foo" pointing to a local $id.

I suspect this fails for the same reason some of the other V2019_09 currently fail, e.g. testRefValidator().

It seems like this PR may already resolve the original issue I ran into (the first test case). It's less obvious how to resolve these next two errors, and whether they are related at all. Could this PR be released, as is, already? Perhaps by splitting up the test cases?

Closes #227

[Whathecode]

I attempted to resolve the remaining failing tests, but am too unfamiliar with the codebase to make this work. Furthermore, the deeper I dig into fixing this, the more I notice this is an unrelated bug. The tests still fail even if the anchors are removed.

I will therefore consider this out of scope for this PR.

While playing around (not committed), I did manage to make the second "$ref": "http://localhost:1234/bar#foo" test pass, by doing a recursive lookup of all containing JsonNodes with $id set and storing it in ValidationContext. This was then retrieved in RefValidator.getRefSchema(). However, this fails for the last test which has a nested non-absolute "$id": "nested.json". Trying to resolve that as a full id was resulting in stack overflows, etc., and made me reach the limit of how much time I want to invest in this, so I won't add anything else to this PR.

[Whathecode]

I filed a new bug report for the $ref issue. This issue is not specific to $anchor, so I suggest to consider it out of scope for this PR.