Fixes #41

[sappenin]

• Adjusts Sha256Condition to have a distinct instance of MessageDigest per-instantiation.
• Adds concurrent unit test coverage to all sub-classes of Sha256Condition to validate that operating on them in parallel does not produce corrupt results.
• Adds some extra JavaDoc to clarify various contracts.
• Misc Formatting

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (development@0ac4aff). Click here to learn what that means. The diff coverage is 66.66%.

@@              Coverage Diff               @@
##             development      #42   +/-   ##
==============================================
  Coverage               ?   37.05%           
  Complexity             ?       90           
==============================================
  Files                  ?       24           
  Lines                  ?      850           
  Branches               ?       86           
==============================================
  Hits                   ?      315           
  Misses                 ?      507           
  Partials               ?       28

Impacted Files	Coverage Δ	Complexity Δ
...ryptoconditions/types/PreimageSha256Condition.java	100% <100%> (ø)	5 <0> (?)
.../interledger/cryptoconditions/Sha256Condition.java	68.42% <60%> (ø)	5 <1> (?)

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