Implemented handshake and heartbeat

Implemented handshake in json server and handoff to endpoint

Endpoint handles connector handshake accept/decline then launches both
its IO threads alongside its heartbeat thread which contains simple
timeout logic, timeout calls connection handles to close, subsequently
killing the endpoint

test/test_core.py

@@ -1,3 +1,5 @@
+import logging
+
 import pytest
 from dynalab_core import Core
 from dynalab_core.config import CoreConfig
@@ -5,14 +7,28 @@ from dynalab_core.errors import CoreStateMismatchError
 from test.common import find_available_port
 
 
-def test_core_cannot_start_twice() -> None:
+def test_core_cannot_start_twice(caplog: pytest.LogCaptureFixture) -> None:
     port = find_available_port(8765)
     core = Core(CoreConfig(port=port))
 
-    core.start()
+    with caplog.at_level(logging.DEBUG, logger="dynalab_core"):
+        core.start()
 
-    try:
-        with pytest.raises(CoreStateMismatchError):
-            core.start()
-    finally:
-        core.stop()
+        try:
+            with pytest.raises(CoreStateMismatchError):
+                core.start()
+        finally:
+            core.stop()
+
+    events = [getattr(record, "event", None) for record in caplog.records]
+    assert events.count("core.starting") == 1
+    assert "core.start_rejected" in events
+    assert "core.stopped" in events
+
+    rejection = next(
+        record
+        for record in caplog.records
+        if getattr(record, "event", None) == "core.start_rejected"
+    )
+    assert rejection.levelno == logging.WARNING
+    assert rejection.core_state == "started"