* Introduce GLOBAL_CHAIN_TYPE and make CHAIN_TYPE thread_local.
This makes testing more explicit and significantly more robust.
* set_local_chain_type() in tests
* cleanup - weird
* get pool tests working with explicit local chain_type config
* core tests working with explicit local chain_type
* p2p tests working with explicit local chain_type
* store tests working
* cleanup, feedback
* 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
