This adds a new Future based, Consumer<Chunk> based, and ability
to control whether or not to generate to the Async Chunk API.
Until Async Chunks merges, these API's are still synchronous, but
this commit will allow plugins to start using the API's in use
with the Async Chunks beta.
The PluginManager incorrectly used synchronization on firing any event
that was marked as synchronous.
This synchronized did not even protect any concurrency risk as
handlers were already thread safe in terms of mutations during event
dispatch.
The way it was used, has commonly led to deadlocks on the server,
which results in a hard crash.
This change removes the synchronize and adds some protection around enable/disable
