switch posix to use signal handlers to kill compute log tail processes
retested, saw builds run fine.
locally, this ran through our unittests faster than the polling version, which waits a polling interval before killing the tail process
Test Plan: bk
Reviewers: #ft, alangenfeld
Reviewed By: #ft, alangenfeld
Differential Revision: https://dagster.phacility.com/D1372