DirectPostingsFormat can minimize binary searching in seek's [LUCENE-4932]

I was looking at the DirectPostingsFormat to see exactly what it's doing when you I seekExact(). It does a plain binary search every time, even if I've positioned the state well into size of the array. A comment (probably left by @mikemccand) shows that he too knows this could be done smarter:

```Java
// TODO: we should use the skip pointers; should be
// faster than bin search; we should also hold
// & reuse current state so seeking forwards is
// faster
```

This issue represents resolving that TODO so it's isn't forgotten.

For inspiration about solving this in a smarter way, see Solr's SortedIntDocSet which avoids binary search using an approach that could be borrowed for this codec.

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Migrated from [LUCENE-4932](https://issues.apache.org/jira/browse/LUCENE-4932) by David Smiley (@dsmiley)

Start by locating DirectPostingsFormat and the seekExact implementation, then read the TODO about skip pointers and reusing current state. Compare the approach with Solr's SortedIntDocSet; done means resolving that TODO so repeated or forward seeks avoid an unnecessary plain binary search while preserving seek behavior.

Written by the indexing model from the issue text.