ei: keep a cache of defunct objects to avoid spamming the log

Because of the asynchronous protocol, we may get this interaction

• server sends A->destroyed()
  • client sends A->foo()
  • client receives A->destroyed()
• server receives A->foo()
• server sends invalid_object_id(A)
  • client receives invalid_object_id(A)

Previously we dropped the object after destroyed() and were thus guaranteed to warn about the invalid object id for that same object later.

Fix this by keeping a cache of defunct object IDs that we know about and ignoring errors for recently dropped objects.

Every 20 ei_dispatch() calls we drop any defunct objects that were unregistered more than 5 seconds ago.

Fixes #49 (closed)

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This sits on top of !269 (merged)

cc @pnowack