Use event sync for increment task launch

piker/data/_buffer.py

@@ -27,6 +27,12 @@ from ._sharedmem import ShmArray
 
 
 _shms: Dict[int, ShmArray] = {}
+_start_increment: Dict[str, trio.Event] = {}
+
+
+def shm_incrementing(shm_token_name: str) -> trio.Event:
+    global _start_increment
+    return _start_increment.setdefault(shm_token_name, trio.Event())
 
 
 @tractor.msg.pub
@@ -47,6 +53,10 @@ async def increment_ohlc_buffer(
     Note that if **no** actor has initiated this task then **none** of
     the underlying buffers will actually be incremented.
     """
+
+    # wait for brokerd to signal we should start sampling
+    await shm_incrementing(shm_token['shm_name']).wait()
+
     # TODO: right now we'll spin printing bars if the last time stamp is
     # before a large period of no market activity.  Likely the best way
     # to solve this is to make this task aware of the instrument's