Fix: several bugs in all Thresholds for log pruning after the refactor which has separated
 log pruning strategy and thresholds

Added method init() to Threshold interface to notify when a new pruning check begins

Add tests for FileSizeThreshold, FileCountThreshold, TransactionTimespanThreshold
Merge pull request #2658 from davidegrohmann/fix-log-pruning-bugs

Fix: several bugs in all Thresholds for log pruning