Re-enable built in auto-save. Fixes BUKKIT-4800

When Minecraft first introduced an auto-save feature, we
were taken by surprise by how much of an impact it actually had on the performance
of the server. After investigating the potential causes of the significant
slow-downs we saw at the time, we came to the conclusion that it was a
combination of the auto-save interval being incredibly frequent and
servers already having an auto-save solution that was conflicting with the
newly added built-in one.

Since we noticed that most servers already had their own auto-save
solution, we decided to completely disable the built in auto-save by
default. In hindsight, however, we were so happy that we discovered and
squashed the cause of the performance issues that we forgot to consider
the future and, as a result, some servers have unfortunately been caught
by surprise when they ran their servers without any auto-save plugins.

Without the auto-save plugin conflict, however, Minecraft's default save
interval of 45 seconds is not suitable for the types of servers that run
Bukkit, to the point where it was negatively impacting performance. As
such, we've decided to re-enable the built in auto-save at an interval of
5 minutes for newly created servers.
Add auto-save plugin redundancy detection.

This change will print a warning when a plugin induces a forced save. A
player or console forcing a save (via a command) is ignored for purposes
of printing a warning.