Add support for signaling an external process via pid file #214

Job	Run time
cache-deps (linux)	15s
build (linux)	31s
unit-test (ubuntu-22.04)	26s
lint (linux)	1m 10s
unit-test (macos-latest)	48s
Build matrix	3s
integration-tests (change-entry-test.sh)	3m 37s
integration-tests (restore-entry-test.sh)	2m 38s
integration-tests (run-go-test.sh)	2m 2s
integration-tests (run-mysql-test.sh)	1m 44s
integration-tests (run-postgres-test.sh)	1m 54s
	15m 8s

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