Add logic for restarting hung catalog application

On occasion, a puppet agent can end up waiting indefinitely on some process that will never return or terminate. Some examples:

• A HTTP connection that was established, but then broken (http_read_timeout defaults to infinity...)
• An I/O read that won't return (for example, networked file system that got interrupted)
• A subprocess that was executed without a timeout and that isn't going to return.
• Module or other plugin code that is susceptible to hangs and contains no defensive timeout or other guard logic.

When this situation occurs, further agent runs will be blocked as the hung run will be holding onto required catalog locks. Often, manual remediation is required to re-start the hung agents.

In situations where hangs can occur often due to transient environment issues (such as flaky networks), it would be useful for the Puppet Daemon to have logic for automatically determining when a hung run should be terminated so that a new one can be started.

For example: if the previous run has been holding the catalog lock for longer than n times the run_interval, kill it start a new one.