Atomic nibble instead of mutex

[betatim]

NibbleStorage uses a big (bad) mutex. This is pretty slow. This PR removes that and instead uses an array of atomic bytes.

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• Did it change the command-line interface? Only backwards-compatible
  additions are allowed without a major version increment. Changing file
  formats also requires a major version number increment.
• For substantial changes or changes to the command-line interface, is it
  documented in CHANGELOG.md? See keepachangelog
  for more details.
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Do the changes respect streaming IO? (Are they
  tested for streaming IO?)

[betatim]

Completely evil :-/

[betatim]

Not sure what to do about this. What is the use case for get_raw_tables() beyond nosy devs wanting to peak inside?

[ctb]

the explicit rationale is somewhere in the merged pull request history... seek answers there :)

[betatim]

Seems like this was the first time this came up: #667 From what I gather of the discussion there get_raw_tables for anything using <8bits per bucket is not what they were expecting to get. Same goes for the Node* classes no?

I don't think we can use the buffer interface to fake the "round up to nearest byte".

Should we move it up the inheritance tree to expose it only for Count* classes?

[standage]

Should we move it up the inheritance tree to expose it only for Count* classes?

I think this is reasonable.

[betatim]

get_raw_tables has been moved and tests adjusted.

Ready for review! @luizirber, @camillescott, @standage , or @ctb

[codecov-io]

Codecov Report

Merging #1601 into master will decrease coverage by <.01%.
The diff coverage is 52.94%.

@@            Coverage Diff            @@
##           master   #1601      +/-   ##
=========================================
- Coverage   70.11%   70.1%   -0.01%     
=========================================
  Files          66      66              
  Lines        8906    8877      -29     
  Branches     3009    2999      -10     
=========================================
- Hits         6244    6223      -21     
- Misses       1040    1041       +1     
+ Partials     1622    1613       -9

Impacted Files	Coverage Δ
khmer/_khmer.cc	57.1% <ø> (ø)	⬆️
lib/hashtable.hh	82.6% <ø> (-0.38%)	⬇️
khmer/_cpy_smallcountgraph.hh	52% <ø> (-2.77%)	⬇️
lib/storage.cc	47.88% <0%> (ø)	⬆️
lib/hashgraph.hh	70.96% <100%> (+0.96%)	⬆️
lib/storage.hh	86.86% <57.14%> (-2.83%)	⬇️

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