[ML] Boost.JSON migration follow-up tasks

[edsavage]

There are a number of non-essential, nice-to-have, items pertaining to the migration to Boost.JSON (#2614 ).

In no particular order:

• [x] TODO in CBoostJsonPoolAllocator::makeStorableDoc: "sort out the allocator pool?" (#2625)
• [x] CStateDecompressor.cc: Remove unneeded static_cast to void *.
• [x] Set sensible limits for maximum values in implementations of SAX style parsers
• [x] Change method names to comply with ml-cpp coding standards.
• [x] CSerializableToJson (and elsewhere): Review exception handling.
• [x] Remove all mentions of RapidJSON throughout
• [x] Better document implementation choices - e.g. need for custom parser in CRetrainableModelJsonReader.cc
• [x] Revisit SAX parser in CStateDecompressor. Could this be made more similar to routine in CJsonStateRestoreTraverser?
• [x] Add unit test for CAnomalyJobConfig::CAnalysisConfig::reparseDetectorsFromStoredConfig - #2623

[edsavage]

The changes in #2648 mop up the remaining loose ends related to the Boost.JSON migration. Closing this issue as now complete.