ARROW-8710: [Rust] Ensure right order of messages written, and flush stream when complete.
ARROW-8710: [Rust] Ensure right order of messages written, and flush stream when complete.

The continuation marker needs to be written before the message byte
length, according to the Arrow specification.

Also for safety's sake the writer needs to be flushed once complete.

Closes #7111 from maxburke/flush_ipc_writer

Authored-by: Max Burke <0706025b2bbcec1ed8d64822f4eccd96314938d0@urbanlogiq.com>
Signed-off-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>

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