Close connection on transform error

[rukai]

Discovered while investigating #706 but quite likely unrelated.

I had previously expected that when a transform returns a Result::Err the connection would be immediately dropped.
Upon writing an integration test to prove this, I discovered that it is in fact not the case.
Currently a transform returning Result::Err will result in no reply sent to the client and the connection remaining open, very likely resulting in the client waiting indefinitely for a response.

So this PR changes shotover to drop the connection immediately when a transform returns a Result::Err.
If a transform wishes to avoid dropping the connection when encountering an error they should handle the error internally.

There was also some changes needed to ensure the send/receive tasks are terminated when the main task is terminated

flushing

I hit an issue where the final messages sent were being lost.
The fix for that was to flush any pending messages before terminating the send task.

This was very tricky to track down because of the connection closing logic needed by cassandra version handling.
It looked like the problem was caused by a bug in that complicated logic, but it was actually that that logic was often exposing the bug due to its timing of message sending -> connection close.