It was incorrectly assumed that a "next" value could not be zero or negative, as this is not representable via the Gamam code. This is incorrect in this case, as we're able to provide a negative offset. Changing to using Integer.MIN_VALUE as indicator that a value is absent instead, as this will never be used.
The priority index documents file can be trivially compressed to a large degree.
Compression schema:
```
00b -> diff docord (E gamma)
01b -> diff domainid (E delta) + (1 + docord) (E delta)
10b -> rank (E gamma) + domainid,docord (raw)
11b -> 30 bit size header, followed by 1 raw doc id (61 bits)
```
The implementation was incorrectly using 1 bit more than it should. The change also adds a put method for Elias delta; and cleans up the interface a bit.
