Fix implementation of heartbeat scheduling

Heartbeat functionality in pedestal uses class Executors
from package java.util.concurrent, its method newScheduledThreadPool
is used to initialize thread pool for sse heartbeat functionality.
However, threads created by default are non-daemon threads where
they really should be rather daemon threads, so they don't prevent
the JVM process from exiting.

My fix supplies ThreadFactory for newScheduledThreadPool, this factory
just sets every jvm thread in the thread pool to be daemon thread.