logmon: Fix a memory leak on task restart
logmon: Fix a memory leak on task restart

Fix a logmon leak causing high goroutine and memory usage when a task
restarts.

Logmon `FileRotator` buffers the task stdout/stderr streams and
periodically flushing them to log files. Logmon creates a new
FileRotator for each stream for each task run. However, the
`flushPeriodically` goroutine is leaked when a task restarts,
holding a reference to a no-longer-needed `FileRotator` instance
along with its 64kb buffer.

The cause is that the code assumed `time.Ticker.Stop()` closes the
ticker channel, thereby terminating the goroutine, but the documentation
says otherwise:

> Stop turns off a ticker. After Stop, no more ticks will be sent. Stop does not close the channel, to prevent a concurrent goroutine reading from the channel from seeing an erroneous "tick".
https://pkg.go.dev/time#Ticker.Stop

hashicorp/nomad,hashicorp/nomad,hashicorp/nomad,hashicorp/nomad,hashicorp/nomad,hashicorp/nomad