[MRG] update gather implementations for LCA and SBT to use thresholding

[ctb]

This is for optimization/performance improvement purposes; ref #838.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[ctb]

My main comment after this experience is that we need some tests to check that threshold_bp is being passed in properly to both LCA and SBT gather... or maybe generically for Index objects...

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@654bf42). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #843   +/-   ##
=========================================
  Coverage          ?   79.34%           
=========================================
  Files             ?       83           
  Lines             ?     7026           
  Branches          ?      469           
=========================================
  Hits              ?     5575           
  Misses            ?     1151           
  Partials          ?      300

Flag	Coverage Δ
#pytests	89.91% <100%> (?)
#rusttests	48.71% <ø> (?)

Impacted Files	Coverage Δ
sourmash/lca/lca_utils.py	96.61% <100%> (ø)

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[luizirber]

Big improvements for LCA search and gather, not much difference for SBTs... (This PR is the gather on the X-axis)

LCA:

SBT:

[ctb]

I think I've addressed all the review issues?