Added ?_trace=1 option to trace SQL

Currently just dumps all SQL statements out on the console.

datasette/tracer.py

@@ -0,0 +1,41 @@
+import asyncio
+from contextlib import contextmanager
+import time
+
+tracers = {}
+
+
+def get_task_id():
+    try:
+        loop = asyncio.get_event_loop()
+    except RuntimeError:
+        return None
+    return id(asyncio.Task.current_task(loop=loop))
+
+
+@contextmanager
+def trace(type, action):
+    task_id = get_task_id()
+    if task_id is None:
+        yield
+        return
+    tracer = tracers.get(task_id)
+    if tracer is None:
+        yield
+        return
+    begin = time.time()
+    yield
+    end = time.time()
+    tracer.append((type, action, begin, end, 1000 * (end - begin)))
+
+
+@contextmanager
+def capture_traces(tracer):
+    # tracer is a list
+    task_id = get_task_id()
+    if task_id is None:
+        yield
+        return
+    tracers[task_id] = tracer
+    yield
+    del tracers[task_id]