Simplify nextFreeRelSlot in ImmDB logic

[nfrisby]

For the file-backed index, the relatively complicated nextFreeRelSlot logic avoids an extra read from disk (although: we're imminently writing to the same page, so does read+write necessarily cost much more than just the write? I don't recall.)

But with the cache, the necessary info is already in memory: nextFreeRelSlot = length currentChunkOffsets - 1.

This could alternatively be added as an assertion, perhaps.

[nfrisby]

length - 1 is the index of the latest offset written to the primary index. But recall: it's initialized to [0] and adding a block B to the index always adds the offset of the next block into the successor of B's slot in the index — that induction suffices.