history: results and tx set iteration is duplicated

[marta-lokhova]

Currently, the same iteration algorithm in the history subsystem is duplicated three times:

• in ApplyCheckpointWork, to get a tx set

  stellar-core/src/catchup/ApplyCheckpointWork.cpp

  Line 136 in fdd833d

  ApplyCheckpointWork::getCurrentTxSet()

• in ApplyChrckpointWork, to get results

  stellar-core/src/catchup/ApplyCheckpointWork.cpp

  Line 179 in fdd833d

  ApplyCheckpointWork::getCurrentTxResultSet()

• in VerifyTxResultsWork

  stellar-core/src/historywork/VerifyTxResultsWork.cpp

  Line 141 in fdd833d

  VerifyTxResultsWork::getCurrentTxResultSet(uint32_t ledger)

This logic is already error-prone enough due to allowed gaps, no reason to duplicate it. We can centralize the algorithm in one function with a little bit of refactoring/templating.