Add methods to prune `KeychainTxOutIndex`

[evanlinjin]

Also:

• Fix KeychainTxOutIndex::store_all_up_to.
• Make KeychainTxOutIndex::derivation_index more efficient.

[codecov-commenter]

Codecov Report

Merging #104 (d62bb54) into master (aabec1b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   51.32%   51.32%           
=======================================
  Files           9        9           
  Lines         602      602           
=======================================
  Hits          309      309           
  Misses        293      293           

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[evanlinjin]

The only things that are left to do here are the "trimming" methods (everything else have been done in some way or another).

However, maybe we should have further discussion on whether these methods are actually useful.

[evanlinjin]

Not needed anymore.