Drop TAILs when connection is dropped

[JLDLaughlin]

This change is 

[benesch]

Awesome! Sorry for the earlier failed suggestions.

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @JLDLaughlin)

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src/coord/command.rs, line 55 at r1 (raw file):

    /// Remove temporary objects created by a given connection.
    TerminateConnectionObjects { conn_id: u32 },

Let's just call this Terminate, to be symmetric with Startup!

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src/pgwire/protocol.rs, line 111 at r1 (raw file):

    // Clean up state tied to this specific connection.
    cmdq_tx
        .clone()

Unnecessary clone, I'll bet?