[MRG] unload data on iteration over SBT leaves

[ctb]

In #1530, we observed high memory usage for sourmash gather --linear against indexed SBTs. This was caused by retention of loaded leaf nodes during the SBT.signatures() method call used by --linear. This PR calls unload_data after each leaf's signature is yielded by the SBT.leaves() method underlying SBT.signatures().

updated benchmarks for 45k indexed zipfile (SBT) with this PR:

	Time	Memory
no-linear/prefetch	10s	215mb
no-linear/no-prefetch	22s	214mb
old linear/prefetch	177s	1502mb
new linear/prefetch	245s	81mb

updated benchmarks for 280k indexed zipfile (SBT) with this PR:

	Time	Memory
no-linear/prefetch	4m 56s	1 GB
no-linear/no-prefetch	1h 16m	1 GB
old linear/prefetch	20m	8.8 GB
new linear/prefetch	18m 46s	1 GB

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Ready for review and merge @luizirber @bluegenes

[codecov]

Codecov Report

Merging #1534 (52710d5) into latest (5234bf1) will increase coverage by 4.98%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           latest    #1534      +/-   ##
==========================================
+ Coverage   90.20%   95.19%   +4.98%     
==========================================
  Files         126       99      -27     
  Lines       21192    17494    -3698     
  Branches     1594     1595       +1     
==========================================
- Hits        19117    16654    -2463     
+ Misses       1844      608    -1236     
- Partials      231      232       +1     

Flag	Coverage Δ
python	95.19% <66.66%> (-0.01%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/sbt.py	80.79% <66.66%> (-0.08%)	⬇️
src/core/src/signature.rs
src/core/src/index/sbt/mod.rs
src/core/src/from.rs
src/core/src/sketch/nodegraph.rs
src/core/src/errors.rs
src/core/src/cmd.rs
src/core/tests/minhash.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/index/mod.rs
... and 18 more

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[bluegenes]

wow, this looks great to me 🚀

[ctb]

thanks!