core/hostname: avoid blocking calls in NMHostnameManager setting static hostname
Of course, blocking and synchronous code is much simpler. But it's also fundamentally wrong to block while we talk to systemd-hostnamed. Refactor to use async operations.
Result: ABORT/NO RESULTS: Test job failure
Passed: 0, Failed: 0, Skipped: 0
Result: STABLE: All tests passed!
M0 PASS: Passed: 592, Failed: 0, Skipped: 31
M1 PASS: Passed: 588, Failed: 0, Skipped: 36
Passed: 1180, Failed 0, Skipped 67.
Executed on: CentOS 8-stream