Define the time unit for reconnect_interval, reconnect_timeout, tombstone_timeout

From https://www.serfdom.io/docs/agent/options.html#reconnect_timeout:

> - reconnect_interval - This controls how often the agent will attempt to connect to a failed node. By default this is every 30 seconds.
> - reconnect_timeout - This controls for how long the agent attempts to connect to a failed node before reaping it from the cluster. By default this is 24 hours.
> - tombstone_timeout - This controls for how long the agent remembers nodes that have gracefully left the cluster before reaping. By default this is 24 hours.

Reading the documentation, it is not clear what unit these parameters expect: seconds? hours?
Would be nice to be include them in the docs.