Centralize and organize I/O-related options into I/O subsystem

Centralize all the I/O related options into the I/O subsystem schema. This includes all XNIO Options, and also includes moving all XNIO worker configuration into the I/O subsystem to be referenced.

The I/O subsystem should define types for each I/O option including (but not limited to):

• send and receive buffer sizes
• TCP nodelay setting
• TCP keepalive
• IP TOS
• Server socket backlog
• Server connection limit low/high water mark
• etc.

Also there should be types defined for:

• XNIO worker thread count
• XNIO worker thread stack size
• XNIO worker thread priority
• etc.

Systems which define I/O entities will reference these types by including the allowed/relevant option types into their schema like the threads subsystem does for thread pool types.

Systems presently defining workers should instead reference a worker defined in the I/O subsystem.