Relax generic types for ConfigurationSerialization

The method signatures are unnecessarily strict for the generic signatures.
This change may cause a compile-time error for extending classes overriding
methods, but no byte signature or compile time call signatures change.
Add ConfigurationSerializable-Serializable compatibility. Adds BUKKIT-4662

This commit adds a comaptibility layer for use between
ConfigurationSerializable and Java Serializable, such that when using the
Bukkit object streams, any ConfigurationSerializable acts as if it
implements Serializable for purposes of that wrapped stream.

Included are a set of unit tests for the stream with a check for backward
compatibility across versions.
[BREAKING] Use event class instead of event for timings. Fixes BUKKIT-4664

TimedRegisteredListener uses a reference to the first event fired. This
causes a memory leak in the server for any references that event has. This
changes TimedRegisteredListener to only store a reference to the class of
the event.

This change is intentionally a breaking change, as it is an obscure part
of the API. A non-breaking change would require the leak to be maintained
or an immediate update for any plugins using the method, as it would be an
indirect break.

A unit test is also included to check behavior of shared superclass
functionality.
Check null before grabbing metadata owning plugin. Fixes BUKKIT-4665

MetadataStoreBase throws a NullPointerException when passed a null value
for setMetaData. The intended behavior is to throw an
IllegalArgumentException. This commit changes the value's null check to
occur before referencing the owning plugin of a value.