fix spurious exit when epoll_wait is interrupted by a signal

[alindima]

I discovered a hang in the pyroscope agent, that is triggered when the Timer thread gets interrupted by a signal.
Instead of retrying the epoll_wait call, the Timer thread simply exits and no data is fed into the server.

This PR checks the epoll_wait error and retries the call if it gets EINTR.

[alindima]

@omarabid @korniltsev can I get a review?

[korniltsev]

I may look into it this week. How do I reproduce? Just SIGSTOP SIGCONT?

[korniltsev]

Nice catch. Thanks for investigation.

[korniltsev]

Not related to the PR and the issue but I noticed timer_fd and epoll_fd are not closed during any error in the epoll thread and seem to be leaking in case of an error.