Fix issues pointed out by nickm.

- Rename tor_tls_got_server_hello() to tor_tls_got_client_hello().
- Replaced some aggressive asserts with LD_BUG logging.

  They were the innocent "I believe I understand how these callbacks
  work, and this assert proves it" type of callbacks, and not the "If
  this statement is not true, computer is exploding." type of
  callbacks.
- Added a changes file.

changes/bug4312

@@ -0,0 +1,11 @@
+  o Security fixes:
+
+    - Block excess renegotiations even if they are RFC5746 compliant.
+      This mitigates potential SSL Denial of Service attacks that use
+      SSL renegotiation as a way of forcing the server to perform
+      unneeded computationally expensive SSL handshakes. Implements
+      #4312.
+
+    - Fix a bug where tor would not notice excess renegotiation
+      attempts before it received the first data SSL record. Fixes
+      part of #4312.