refactoring ops callback hell, secure io thread from custom gotData()/transcoder/decode errors,
extract base class for SingleKey get operations.

Motivation
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- a lot of code duplication in callbacks, custom callbacks for sim. ops with same int
- any exception in gotData/decode() drops down memcached connection, instead of
  working out certain request error handling.

Modifications
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- new common DataCallback
- new base class SingleGetOperationImpl / Exp
- all custom callbacks removed from single data ops
- try/catch to gotData calls

Result
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Clean callback impl and secured decoding.