Refactor data migration to lazily load results

[prd-fox]

Refactor data migration to lazily load results as to not blow memory.
The loaders now return one result at a time.
Removed dud test files and service definitions that were not used.

Fixes #705

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7236fe9). Click here to learn what that means.
The diff coverage is 99.41%.

@@            Coverage Diff            @@
##             master     #719   +/-   ##
=========================================
  Coverage          ?   99.48%           
  Complexity        ?     1894           
=========================================
  Files             ?      286           
  Lines             ?     5619           
  Branches          ?      285           
=========================================
  Hits              ?     5590           
  Misses            ?        5           
  Partials          ?       24

Impacted Files	Coverage Δ	Complexity Δ
...om/quorum/tessera/data/migration/JdbcCallback.java	100% <ø> (ø)	1 <0> (?)
...m/tessera/data/migration/StoreLoaderException.java	100% <ø> (ø)	1 <0> (?)
...om/quorum/tessera/data/migration/SqliteLoader.java	100% <100%> (ø)	4 <3> (?)
...quorum/tessera/data/migration/CmdLineExecutor.java	100% <100%> (ø)	6 <5> (?)
...um/tessera/data/migration/DataExporterFactory.java	100% <100%> (ø)	3 <0> (?)
...uorum/tessera/data/migration/JdbcDataExporter.java	100% <100%> (ø)	4 <1> (?)
.../quorum/tessera/data/migration/H2DataExporter.java	100% <100%> (ø)	4 <2> (?)
...a/com/quorum/tessera/data/migration/DataEntry.java	100% <100%> (ø)	3 <3> (?)
...rum/tessera/data/migration/DirectoryStoreFile.java	100% <100%> (ø)	5 <4> (?)
...rum/tessera/data/migration/SqliteDataExporter.java	100% <100%> (ø)	6 <1> (?)
... and 3 more

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