Zeek's analyzer API makes it hard to determine during analyzer
shutdown whether a regular end-of-data has been reached, or if we're
aborting in the middle of a session (e.g., because Zeek missed the
remaining packets): the corresponding analyzer method, `EndOfData()`
gets called in both cases.
In an earlier change, we had stopped signaling Spicy analyzers a
regular finish when that `EndOfData()` method executes, because doing
so could trigger a parse error if it wasn't a regular shutdown—-which
isn't desired, a user request was to just silently stop processing in
this case.
However, that behavior now seems unfortunate in the case that one
deliberately calls `zeek::protocol_handle_close()` to terminate an
analyzer: this feels like a regular shutdown that should just
immediately happen. We achieve this now in this function by
additionally signaling the shutdown at the TCP layer as an "end of
file", which, for Spicy analyzers, happens to run the final, orderly
tear-down.
Not exactly great, but ti seems to thread the needle to achieve the
desired semantics in both cases.
