First draft aggregate feedz support

Orient shm-flow-arrays around the new idea of a `Flume` which provides
access, mgmt and basic measure of real-time data flow sets (see water
flow management semantics).

- We discard the previous idea of a "init message" which contained all
  the shm attachment info and instead send a startup message full of
  `Flume.to_msg()`s which are symmetrically loaded on the caller actor
  side.

- Create data-flows "entries" for every passed in fqsn such that the consumer gets back
  streams and shm for each, now all wrapped in `Flume` types. For now we
  allocate `brokermod.stream_quotes()` tasks 1-to-1 for each fqsn
  (instead of expecting each backend to do multi-plexing, though we
  might want that eventually) as well a `_FeedsBus._subscriber` entry
  for each. The pause/resume management loop is adjusted to match.
  Previously `Feed`s were  allocated 1-to-1 with each fqsn.

- Make `Feed` a `Struct` subtype instead of a `@dataclass` and move all
  flow specific attrs to the new `Flume`:
  - move `.index_stream()`, `.get_ds_info()` to `Flume`.
  - drop `.receive()`: each fqsn entry will now require knowledge of
    separate streams by feed users.
  - add multi-fqsn tables: `.flumes`, `.streams` which point to the
    appropriate per-symbol entries.

- Async load all `Flume`s from all contexts and all quote streams using
  `tractor.trionics.gather_contexts()` on the client `open_feed()` side.

- Update feeds test to include streaming 2 symbols on the same (binance)
  backend.

tests/test_feeds.py

@@ -5,6 +5,7 @@ Data feed layer APIs, performance, msg throttling.
 from pprint import pprint
 
 import pytest
+import tractor
 import trio
 from piker import (
     open_piker_runtime,
@@ -16,7 +17,7 @@ from piker.data import ShmArray
 @pytest.mark.parametrize(
     'fqsns',
     [
-        ['btcusdt.binance']
+        ['btcusdt.binance', 'ethusdt.binance']
     ],
     ids=lambda param: f'fqsns={param}',
 )
@@ -30,7 +31,13 @@ def test_basic_rt_feed(
     '''
     async def main():
         async with (
-            open_piker_runtime('test_basic_rt_feed'),
+            open_piker_runtime(
+                'test_basic_rt_feed',
+                # XXX tractor BUG: this doesn't translate through to the
+                # ``tractor._state._runtimevars``...
+                registry_addr=('127.0.0.1', 6666),
+                debug_mode=True,
+            ),
             open_feed(
                 fqsns,
                 loglevel='info',
@@ -42,24 +49,38 @@ def test_basic_rt_feed(
 
             ) as feed
         ):
+            # verify shm buffers exist
             for fqin in fqsns:
-                assert feed.symbols[fqin]
+                flume = feed.flumes[fqin]
+                ohlcv: ShmArray = flume.rt_shm
+                hist_ohlcv: ShmArray = flume.hist_shm
 
-            ohlcv: ShmArray = feed.rt_shm
-            hist_ohlcv: ShmArray = feed.hist_shm
+            quote_count: int = 0
+            stream = feed.streams['binance']
+            async for quotes in stream:
+                for fqsn, quote in quotes.items():
 
-            count: int = 0
-            async for quotes in feed.stream:
+                    # await tractor.breakpoint()
+                    flume = feed.flumes[fqsn]
+                    ohlcv: ShmArray = flume.rt_shm
+                    hist_ohlcv: ShmArray = flume.hist_shm
 
                     # print quote msg, rt and history
                     # buffer values on console.
-                pprint(quotes)
-                pprint(ohlcv.array[-1])
-                pprint(hist_ohlcv.array[-1])
+                    rt_row = ohlcv.array[-1]
+                    hist_row = hist_ohlcv.array[-1]
+                    # last = quote['last']
 
-                if count >= 100:
+                    # assert last == rt_row['close']
+                    # assert last == hist_row['close']
+                    pprint(
+                        f'{fqsn}: {quote}\n'
+                        f'rt_ohlc: {rt_row}\n'
+                        f'hist_ohlc: {hist_row}\n'
+                    )
+                    quote_count += 1
+
+                if quote_count >= 100:
                     break
 
-                count += 1
-
     trio.run(main)