contrib/linearize: Guarantee that output is generated in-order

This was typically ensured implicitly by virtue of normal bitcoind
operation.  Adding an explicit check provides a stronger guarantee, and
it is cheap to add.
contrib/linearize: split block files based on year-month, not just year
contrib/linearize: Add feature to set file's timestamp based on block header time.
Merge pull request #4762

399cdbc contrib/linearize: Add feature to set file's timestamp based on block header time. (Jeff Garzik)
8f5a423 contrib/linearize: split block files based on year-month, not just year (Jeff Garzik)
75400a2 contrib/linearize: Guarantee that output is generated in-order (Jeff Garzik)