ABC: add stub AccountDatabaseAPI.lock_changes().

SQUASHED:

lint/mypy,ABC: silence error in an AccountDatabaseAPI lacking a stub.

The error is:

    eth/vm/state.py:180: error: "AccountDatabaseAPI" has no attribute "lock_changes"

The stub comment is copied over from AccountDatabaseStorageAPI,
removing the reference to "storage".

ABC: improve AccountDatabaseAPI.lock_changes() heredoc.

Co-authored-by: Jason Carver <ut96caarrs@snkmail.com>

Author: veox

eth/abc.py

@@ -1945,6 +1945,22 @@ def commit(self, checkpoint: JournalDBCheckpoint) -> None:
         """
         ...
 
+    @abstractmethod
+    def lock_changes(self) -> None:
+        """
+        Locks in changes across all accounts' storage databases.
+
+        This is typically used at the end of a transaction, to make sure that
+        a revert doesn't roll back through the previous transaction, and to
+        be able to look up the "original" value of any account storage, where
+        "original" is the beginning of a transaction (instead of the beginning
+        of a block).
+
+        See :meth:`eth.abc.AccountStorageDatabaseAPI.lock_changes` for
+        what is called on each account's storage database.
+        """
+        ...
+
     @abstractmethod
     def make_state_root(self) -> Hash32:
         """