Improve Teleport reload behavior

[espadolini]

This PR tweaks some of the Teleport behavior around SIGHUP reloads and forking:

• the PID file (if any) is not recreated after a host CA rotation (which spins up a new TeleportProcess in the same process)
• the PID file (if any) is created atomically, and the process that creates it holds an exclusive advisory lock on it that only unlocks at process end (so that we can pkill -L -F <pidfile> and greatly reduce the chance that we accidentally send a signal to an unintended process
• systemd unit files are configured to use pkill -L -F on the pidfile rather than relying on the $MAINPID as detected by systemd: this is needed because the way Teleport forks a new subprocess on HUP to upgrade itself leaves systemd with no way to accurately keep track of what the "main" Teleport process should be, with the end result being that a second systemctl reload while the first Teleport process hasn't exited yet is likely to not get directed at the new Teleport process (weirdly enough, after the second SIGHUP systemd seems to reload the PIDFile configured in the unit 🤷)
• child Teleport processes are actively reaped on death even after exiting the WaitForSignals loop; this prevents the accumulation of zombies while Teleport is gracefully shutting down (possible when reloading more than once, which is plausible with a combination of very long lived sessions and frequent updates)
• failing to start a new child process doesn't leave the parent process hanging for two minutes while not responding (but buffering) signals
• exponential backoff is added to the logging of "Waiting for services: [...] to finish." and "Shutdown: waiting for N connections to finish.", so that slow graceful shutdowns don't fill the log too much

Changelog: improved the stability of Teleport during graceful upgrades

[EdwardDowling]

This looks good to me but is not a part of Teleport I am familiar with with.

[public-teleport-github-review-bot]

@espadolini See the table below for backport results.

Branch	Result
branch/v15	Create PR