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ARROW-6480: [Crossbow] Summary report e-mailer with polling logic

This is a rebased version of #5319. Crossbow had a major refactoring in #5296 so the two implementation have diverged.

Closes #5332 from kszucs/crossbow-report and squashes the following commits:

ed62de438 <Krisztián Szűcs> unused import
76a3269ae <Krisztián Szűcs> share output redirection; use pass_obj instead of pass_context
9a0069cf0 <Krisztián Szűcs> start incrementing from 0
16015d227 <Krisztián Szűcs> merge tests.yaml with tasks.yaml
00f576629 <Krisztián Szűcs> latest-prefix
162a0fbf8 <Krisztián Szűcs> polling
1a8790f89 <Wes McKinney> Add polling logic to wait for job completion
6aa2a7eca <Wes McKinney> Test disabling stop_if_pending
3e4580aed <Wes McKinney> Implement e-mail report for a Crossbow job

Lead-authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Co-authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>