Simplify positions

[fulmicoton]

Right now the positions of all terms are all encoded in the same position stream, delta and bitpacked.
The last block is simply padded.

We want to move from this to something closer to what is done for doc ids.
Each term would be encoded independently and the last block would be vint encoded.

This should unlock the possibility to use phrase query on "distant directory".

This means

• encode/read the byte range in TermInfo
• encode/read the new positions info.
• limit the fetch on positions to our tight range
• check for possible performance hits

Positions encoding:
Assuming the overall number of positions is P

- Positions: NumBitPackedBlocks BitPackedPositionBlock^(P/128) BitPackedPositionsDeltaBitWidth VIntPosDeltas?
- NumBitPackedBlocks: VInt = P / 128
- BitPackedPositionBlock: bit width encoded block of 128 delta pos
- BitPackedPositionsDeltaBitWidth: (BitWidth: u8)^NumBitPackedBlocks
- VIntPosDeltas: (VIntPosDelta)^(P % 128).