ARROW-17060: [C++] Change AsOfJoinNode to use ExecContext's Memory Pool

[iChauster]

Change asof_join_node.cc so the Builder it uses allocates memory from the ExecContext's memory pool instead of the default memory pool.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-17060

[github-actions]

⚠️ Ticket has not been started in JIRA, please click 'Start Progress'.

[ursabot]

Benchmark runs are scheduled for baseline = a1cd655 and contender = b71e503. b71e503 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️1.95% ⬆️0.85%] test-mac-arm
[Failed ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️1.43% ⬆️0.54%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] b71e5031 ec2-t3-xlarge-us-east-2
[Finished] b71e5031 test-mac-arm
[Failed] b71e5031 ursa-i9-9960x
[Finished] b71e5031 ursa-thinkcentre-m75q
[Finished] a1cd655f ec2-t3-xlarge-us-east-2
[Failed] a1cd655f test-mac-arm
[Failed] a1cd655f ursa-i9-9960x
[Finished] a1cd655f ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java