Add config to keep some extra days of data

[coderofstuff]

Notes

• New configuration usage --keep-extra-days-data=N - set this to some number to keep that much extra data before the pruning point.
• Implementation detail - this goes through the selected parent chain to find the first block that is past the N days threshold. There is no extra logic that ensures the anticone of the last SPC block found is also retained as such those will still be pruned.