[FlightRPC] Fix auth incompatibilities between Python/Java

[asfimport]

It turns out the blocking-forever issue was a combination of problems in Python and Java. We should simply fix the issues.

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The Flight Handshake method can be insecure, and currently has a surprising failure mode; we should document these caveats (blocks forever waiting on client/server; insecure depending on deployment configuration)

Reporter: David Li / @lidavidm
Assignee: David Li / @lidavidm

PRs and other links:

• GitHub Pull Request #4838

_{Note: This issue was originally created as ARROW-5877. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 4838
#4838