Indexing: Handle LogAddressIndex queries via an IndexProvider

[froch]

We want to be able to find and serve LogAddressIndex indexes

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Acceptance Criteria:

• [x] create a LogAddressIndexProvider conforming to the bstream.IndexProvider interface
• [x] determine if a given block number is within an index
• [x] determine if a given block number matches an element in an index
• [x] fetch and return all next matching indexes for a given block number

[froch]

Whiteboard of the flow for querying an IndexProvider

[sduchesneau]

bstream branch stepd/index-provider contains the (almost-complete) code for querying the logAddressIndex from the IrrIndexProvider... names will have to be reworked in that last one to prevent confusion.