Disable `greenback` sync fn breakpointing by def

Opting for performance over broad multi-actor "debug-ability" from
sync-function-contexts when `debug_mode=True` is set;
IOW prefer no behind-the-scenes `greenlet` perf impact over being
able to use an actor-safe `breakpoint()` wherever as per,
https://greenback.readthedocs.io/en/latest/principle.html#performance

Adjust the breakpoint restore ex script to match.

examples/debugging/restore_builtin_breakpoint.py

@@ -19,8 +19,11 @@ async def main() -> None:
     async with tractor.open_nursery(
         debug_mode=True,
         loglevel='devx',
-    ) as an:
-        assert an
+        maybe_enable_greenback=True,
+        # ^XXX REQUIRED to enable `breakpoint()` support (from sync
+        # fns) and thus required here to avoid an assertion err
+        # on the next line
+    ):
         assert (
             (pybp_var := os.environ['PYTHONBREAKPOINT'])
             ==

tractor/_root.py

@@ -166,7 +166,9 @@ async def open_root_actor(
 
     # enables the multi-process debugger support
     debug_mode: bool = False,
-    maybe_enable_greenback: bool = True,  # `.pause_from_sync()/breakpoint()` support
+    maybe_enable_greenback: bool = False,  # `.pause_from_sync()/breakpoint()` support
+    # ^XXX NOTE^ the perf implications of use,
+    # https://greenback.readthedocs.io/en/latest/principle.html#performance
     enable_stack_on_sig: bool = False,
 
     # internal logging