increase length check in _unpickle_context

The length check len(name) < 100 fails if longer hostnames are used.
A hostname may be 255 bytes long, in addition with ssh as transport and if a bastion host is used this may result in a 518 character long string (ssh.<bastionhost>.ssh.<targethost> ~max 4 + 255 +4 + 255 ).
So the length check should be increased to allow max 518 character.

mitogen/core.py

@@ -2309,7 +2309,7 @@ class Context(object):
 def _unpickle_context(context_id, name, router=None):
     if not (isinstance(context_id, (int, long)) and context_id >= 0 and (
         (name is None) or
-        (isinstance(name, UnicodeType) and len(name) < 100))
+        (isinstance(name, UnicodeType) and len(name) < 519))
     ):
         raise TypeError('cannot unpickle Context: bad input')