feat: add a method for resetting index state

[EvanHahn]

I recommend reviewing this with whitespace changes disabled.

This adds MultiCoreIndexer.prototype.removeCoreAndUnlinkIndexStorage(core) which removes the core from the indexer and destroys the storage.

This felt a bit complicated, so I'm keen to hear feedback on that in particular (but also anything else!).

Addresses #26.

[EvanHahn]

tl;dr: this is slower but I think this is fine, as long as we don't have tons of cores.

Something needed to change here. If you remove a stream, we might need to change the state of drained; a simple boolean is insufficient for this purpose.

A few options I considered:

1. Make the value computed and remove state. Pros: reduces the amount of state that can go wrong. Con: a performance cost; the operation goes from O(1) to O(N).
2. Make the state more complex: probably a Set<CoreIndexStream> of un-drained streams. Pros: roughly as fast as the previous approach. Con: requires more delicate state management.
3. Make the state computed, but cache the value and mark it "dirty". This would probably mean that #drained was of type undefined | boolean. Pros: a hybrid of the previous approaches. Cons: still incurs a performance hit the first time you read.

I went with the first option because I felt it was the simplest, but option 2 also seems reasonable.

If there are a small number of cores, this is probably not an issue. That's confirmed by the benchmarks/multi-core-indexer.js benchmark:

Before	After
# Index 20 cores of 1000 blocks (10 times) ok ~4.57 s (4 s + 566600718 ns)	# Index 20 cores of 1000 blocks (10 times) ok ~4.6 s (4 s + 599290806 ns)

However, if there are lots of cores, this might become a problem.

Is what I have here okay?

[EvanHahn]

Decided to take a different approach with #34.