rebased https://github.com/ethereum/go-ethereum/pull/29766 . The
downstream branch appears to have been deleted and I don't have perms to
push to that fork.
`TerminalTotalDifficultyPassed` is removed. `TerminalTotalDifficulty`
must now be non-nil, and it is expected that networks are already
merged: we can only import PoW/Clique chains, not produce blocks on
them.
---------
Co-authored-by: stevemilk <wangpeculiar@gmail.com>
* eth: drop support for forward sync triggers and head block packets
* consensus, eth: enforce always merged network
* eth: fix tx looper startup and shutdown
* cmd, core: fix some tests
* core: remove notion of future blocks
* core, eth: drop unused methods and types
