Remove use of daemon threads, fixing potential resource leak

[fractalwrench]

Goal

Users have reported that daemon threads are not stopped when an application is stopped in a standalone Tomcat server. In an embedded server such as Spring Boot this does not matter as the JVM will always terminate when the application server is brought down, but in a standalone server, this can lead to threads leaking and performance problems over time.

We should fix this behaviour so that threads terminate properly when the application is shut down.

Changeset

• Removed DaemonThreadFactory
• Updated sessionExecutorService to use a regular thread factory, whose threads will be stopped on application shutdown
• Refactored all shutdown logic to a close() method
• Made Bugsnag implement Closable
• Called shutdownNow on the sessionExecutorService, as the periodic check to flush sessions is no longer needed when the application is terminating.

Tests

Installed a standalone Tomcat server locally, and deployed a plain Spring application integrated with Bugsnag. Started then stopped the Tomcat server, and inspected the logs, verifying that there were no messages about thread leaks when this changeset was applied.

I also verified that the application shutdown when a crash occurred on startup, to verify that the scenario in #121 has not resurfaced.

[simonbowring]

This looks good to me.

Incidentally, as a consequence of this PR and #121, in our example apps and test fixtures (I count 4 occurrences) we no longer need to do:

System.exit(SpringApplication.exit(...));

Instead we can just do this to exit an application without the application hanging:

SpringApplication.exit(...);