* op-node: Add option to enable safe head history database.

Currently it just logs the information and has no actual database.

* op-node: Introduce pebble db to store safe head updates

* op-node: Reset the pipeline if safe head updates fail to be recorded

* go mod tidy

* op-node: Truncate when L1 head is reduced.

* op-node: Record accurate safe head data when restarting

* op-node: Ensure the latest safe head update is reset on a pipeline reset.

* op-node: Improve thread safety of safedb

* op-node: Add L2 block number to stored data

* op-node: Add API method to retrieve safe head at an L1 block number

* op-node: Tidy up key handling in safedb.

* op-node: Use an explicit reset event to clear entries invalidated by a pipeline reset

Add action test to confirm reorgs are correctly handled.

* op-node: Undo changes to start. We always step the safe head back at least one block.

* op-node: Simplify error message when requested record is prior to start of history

* op-node: Improve log message

* op-node: Tidy up

* op-node: Include L1 block number in response.

* op-node: Add missing AssertExpectations

* op-node: Verify key prefix