Add useful toString()s to DocValues impls [LUCENE-5095]

Currently, you have no insight into what these are doing (what kind of compression does your numerics have, how much RAM are the datastructures taking up, so on.

So I think we should add some cheap toString's (nothing heavy), things like

```Java
`@Override`
public String toString() {
return "Lucene42Numeric(compression=TABLE,bpv=" + bitsPerValue)";
}
```

and so on. Maybe we could think about printing them from CheckIndex when verbose=true too.

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Migrated from [LUCENE-5095](https://issues.apache.org/jira/browse/LUCENE-5095) by Robert Muir (@rmuir), 2 votes, updated Jul 07 2013

Review the Java DocValues implementations and the issue's proposed Lucene42Numeric example first. Identify the relevant implementations and determine which inexpensive compression, bits-per-value, or memory details each can expose. Done means useful lightweight toString() output across the implementations, with CheckIndex verbose output considered only if the scope is confirmed.

Written by the indexing model from the issue text.