demonstrate syncing with block_position indexing

[rajarshimaitra]

This is an attemted approach at #85 . Not final and can be structured better. Opening up for approach discussions.

Build on top of #77 and demos it over electrum API.

Things Done:

• new BlockPosition struct that is ChainPosition with custom ordering.
• Fetch block position data from electrum and create the index accordingly.
• New SyncBlockPosition command to sync in this way. This will create a new tracker and a new db. the regular TxHeight tracker and db won't be updated in that call.

Question:

• What are better way to structure this. There's a lot of copy past of existing code, and I couldn't find a good way to satisfy type checker, with the way the main function is structured currently. So just dumped all the code in a separate cli arg.

[codecov-commenter]

Codecov Report

Merging #119 (3cf3831) into master (e9ac33e) will decrease coverage by 4.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   65.30%   61.14%   -4.16%     
==========================================
  Files           8        8              
  Lines         294      314      +20     
==========================================
  Hits          192      192              
- Misses        102      122      +20     

Impacted Files	Coverage Δ
bdk_chain/src/chain_data.rs	35.10% <0.00%> (-9.49%)	⬇️

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[LLFourn]

Can you try and re-do this PR on top of master @rajarshimaitra?

[rajarshimaitra]

@LLFourn rebased on master..