Remove stream opening lock on `DataFeed`

Fixes to `tractor` that resolve issues with async generators being
non-task safe make the need for the mutex lock in
`DataFeed.open_stream()` unnecessary. Also, don't bother pushing empty
quotes from the publisher; avoids hitting the network when possible.

piker/brokers/data.py

@@ -1,5 +1,5 @@
 """
-Live data feed machinery
+Real-time data feed machinery
 """
 import time
 from functools import partial
@@ -9,7 +9,11 @@ import socket
 import json
 from types import ModuleType
 import typing
-from typing import Coroutine, Callable, Dict, List, Any, Tuple
+from typing import (
+    Coroutine, Callable, Dict,
+    List, Any, Tuple, AsyncGenerator,
+    Sequence,
+)
 import contextlib
 from operator import itemgetter
 
@@ -77,7 +81,7 @@ async def stream_quotes(
     get_topics: typing.Callable,
     get_quotes: Coroutine,
     feed: BrokerFeed,
-    rate: int = 5,  # delay between quote requests
+    rate: int = 3,  # delay between quote requests
     diff_cached: bool = True,  # only deliver "new" quotes to the queue
 ) -> None:
     """Stream quotes for a sequence of tickers at the given ``rate``
@@ -135,10 +139,11 @@ async def stream_quotes(
                     # quote).
                     new_quotes.setdefault(quote['key'], []).append(quote)
         else:
-            log.info(f"Delivering quotes:\n{quotes}")
+            # log.debug(f"Delivering quotes:\n{quotes}")
             for quote in quotes:
                 new_quotes.setdefault(quote['key'], []).append(quote)
 
+        if new_quotes:
             yield new_quotes
 
         # latency monitoring
@@ -341,16 +346,20 @@ class DataFeed:
         self._quote_type = None
         self._symbols = None
         self.quote_gen = None
-        self._mutex = trio.StrictFIFOLock()
         self._symbol_data_cache: Dict[str, Any] = {}
 
-    async def open_stream(self, symbols, feed_type, rate=1, test=None):
+    async def open_stream(
+        self,
+        symbols: Sequence[str],
+        feed_type: str,
+        rate: int = 1,
+        diff_cached: bool = True,
+        test: bool = None,
+    ) -> (AsyncGenerator, dict):
         if feed_type not in self._allowed:
             raise ValueError(f"Only feed types {self._allowed} are supported")
 
         self._quote_type = feed_type
-
-        async with self._mutex:
         try:
             if self.quote_gen is not None and symbols != self._symbols:
                 log.info(
@@ -384,6 +393,7 @@ class DataFeed:
                     broker=self.brokermod.name,
                     symbols=symbols,
                     feed_type=feed_type,
+                    diff_cached=diff_cached,
                     rate=rate,
                 )