We recently ran into a core dump issue due to one of our TransactionPlugin's continuations being called back with TXN_CLOSE event. The current CPPAPI's TransactionPlugin does not expect a call back on TXN_CLOSE, and asserts on it. This is because the expectation seems to be that all the continuations for TransactionPlugins attached to the Transaction are destroyed in the global TXN_CLOSE hook. However, this only works based on the plugin table in the Transaction object, which gets populated based on `Transaction::addPlugin()`. Turns out, some of our plugins do not call `Transaction::addPlugin()` and just create a `TransactionPlugin()` and register hooks to it resulting in them getting called back for TXN_CLOSE event as well. Long story short, it didn't particularly seem unreasonable to add support to TXN_CLOSE event in TransactionPlugin, since the current API does not seem to mandate calling `Transaction.addPlugin()`.
https://github.com/apache/trafficserver/pull/6800 Please let me know if there are any concerns/comments.
