* maintain header_head as distinctly separate from head
* cleanup corrupted storage log msg
* simplify process_header and check_header_known
* remember to commit the batch when successfully processing a header...
* rework sync_block_headers for consistency with process_block_header
* cleanup unrelated code
* fix pool tests
* cleanup chain tests
* cleanup chain tests (reuse helpers more)
* cleanup - head not header on an extension
shortcircuit "rewind and apply fork" for headers if next header
