More centralisation of the I/O manager hooks

This is a direct follow-on from MR !4694 (closed). It picks up the next planned steps:

It includes

• introducing a per-capability I/O manager structure
• moving the {blocked,sleeping}_queue from being global vars in the scheduler to being in the per-cap I/O manager structure
• clarify the checks used in the scheduler for whether I/O or timers are pending
• merge the AwaitEvent.h into IOManager.h
• pass the cap explicitly in various places rather than referring to MainCapability all over the place