Remove index table

[aalda]

We are using the index table to map from event hashes to history tree versions, but that responsibility should be exclusive of the hyper tree, given that now, it stores the raw version in the shortcut leaves.

In this manner, we could eliminate the need for using another table to support fast mappings. With this change, every membership operation must query first the hyper tree before generating the audit path from the history tree, and thus, incurs in a latency penalty. However, given that the hyper tree is the only one that holds a lock for queries, in theory, it shouldn't reduce balloon's throughput.

This change helps to reduce space and write amplification in storage.

[aalda]

These are the results obtained in the microbenchmarks before and after removing the index table. The benchmarks execute 1,000,000 operations in every run. The results are shown in nanoseconds per operation.

	Add (ns/op)	Sequential Query (ns/op)	Parallel Query - 10 (ns/op)	Parallel Query - 100 (ns/op)
Without index	97773	213293	139587	96872
With index	101912	110294	92270	106911

We can observe that sequential queries almost duplicate latencies when removing the index table. This result was the expected given that we are now quering both trees (hyper and history) sequentially. However, as we increase parallelism, it gets better results. It also slightly improves Add latencies.