Convert LogAddressIndex to a generic BlockIndex

[froch]

We want to convert the work on LogAddressIndex to a generic BlockIndex.

Ideally, in a second step, the LogAddressIndex could be rewritten to piggyback on the generic BlockIndex/indexer/provider

The idea is to not have multiple time the same code that : "defines the boundaries of the index, encodes/decode simple KV to RoraringBitmap in protobuf + lowBoundary parsing in the name, indexSizes management...."

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Acceptance Criteria:

• [x] I can create a generic indexer with NewBlockIndexer(...)
• [x] I can call the generic indexer to inform of the content or null-content of a block (which keys are of interest in this block)
• [x] The generic indexer will create the correctly bounded index files in the store
• [x] A generic Index type for encoding/decoding (used by indexer and provider)
• [ ] A provider that implements the Matching(), NextMatching, etc. calls from the IndexProvider interface, given a few methods that we pass on the creation of the provider ---- OR ----- a provider library that makes this work easy to implement... we want to abstract the logic of looking for index files, parsing them, reading the next one, grabbing the roaring bitmaps for given keys at given block heights....

[sduchesneau]

as alexandre, I want to be able, while processing blocks with some machinery, to mark blocks as interesting for a given key, and have a GenericIndexer store that info in indexes that will be used after.

Example:

• I read block 23, it contains interesting values for keys: bob, alice
• I call GenericIndexer.Add([bob, alice], 23)
• I read block 24, it contains nothing interesting, I call GenericIndexer.Skip(24)
• When I get to block 100, the generic indexer discards all info before 100 becaue it does not contain a boundary.
• When I get to block 200, the generic indexer creates a file 00000100.alexfunstuff.100.idx where 00000100 is the lower block, the other 100 is the size.

The constructhor needs the following info:

• dstore.Store
• a short name (ex: alexfunstuff)
• the target index size (ex: 1000) -- NOTE that we have a ""similar"" implementation in irreversible indexer, that is able to produce multiple index sizes. it would be cool to support this, but not necessary.

[froch]

tagging @abourget per request