Make fdbmonitor restartable in the debian package

The pull request https://github.com/apple/foundationdb/pull/3841 makes fdbmonitor restartable in rpm:
1. All chld subprocesses are terminated when fdbmonitor is killed.
2. Systemd restarts fdbmonitor authomatically in 60s

This issue is to make the same in debian

Start by reading pull request #3841 and comparing its RPM changes with the Debian package configuration. Verify that child processes are terminated when fdbmonitor is killed and that systemd restarts it automatically after 60 seconds; the Debian package should provide the same behavior.

Written by the indexing model from the issue text.