Introduces IO abstractions apart from asyncio or those attached to IRawConnection

Also adds `msgio` utilities to mirror the Go implementation

libp2p/io/utils.py

@@ -0,0 +1,21 @@
+from libp2p.io.abc import Reader
+from libp2p.io.exceptions import IncompleteReadError
+
+DEFAULT_RETRY_READ_COUNT = 100
+
+
+async def read_exactly(
+    reader: Reader, n: int, retry_count: int = DEFAULT_RETRY_READ_COUNT
+) -> bytes:
+    """
+    NOTE: relying on exceptions to break out on erroneous conditions, like EOF
+    """
+    data = await reader.read(n)
+
+    for _ in range(retry_count):
+        if len(data) < n:
+            remaining = n - len(data)
+            data += await reader.read(remaining)
+        else:
+            return data
+    raise IncompleteReadError({"requested_count": n, "received_count": len(data)})