ARROW-17060: [C++] Change AsOfJoinNode to use ExecContext's Memory Pool (#13585)
ARROW-17060: [C++] Change AsOfJoinNode to use ExecContext's Memory Pool (#13585)

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

Authored-by: Ivan Chau <072f5c5de38ee2566abbfaec5a29ae99aad53dbd@gmail.com>
Signed-off-by: Weston Pace <0dd0706858a011bb30a3fd82b71cee4f9094c502@gmail.com>
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