Switch shutdown to use fast shutdown

[boustrophedon]

• SIGKILL causes shared memory and segfaults to leak https://www.postgresql.org/docs/current/server-shutdown.html
• Just using SIGTERM everywhere creates deadlock conditions when used with connection poolers in some cases, but it would be nice to have a graceful shutdown option eventually.
• Ended up using SIGINT fast shutdown mode, which actively terminates connections rather than waiting for them to complete.
• This is technically a breaking change because using the persist option does not using graceful shutdown anymore.

[coveralls]

Pull Request Test Coverage Report for Build 11006865061

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 24 of 25 (96.0%) changed or added relevant lines in 1 file are covered.
• 14 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-1.2%) to 84.902%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib.rs	24	25	96.0%

Files with Coverage Reduction	New Missed Lines	%
src/daemon.rs	14	77.55%

Totals
Change from base Build 10434873697:	-1.2%
Covered Lines:	478
Relevant Lines:	563

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💛 - Coveralls