Refactor Reactor into a more generic IO-with-timeout monitor,
using a Queue to simplify the implementation.
Move request-buffering logic into Server#reactor_wakeup.
Fixes bug in managing timeouts on clients.
Move, update and rewrite documentation to match updated class structure.

- In `Reactor#shutdown`, `@selector` can be closed before the call to `#wakeup`, so catch/ignore the `IOError` that may be thrown.
- `Reactor#wakeup!` can delete elements from the `@timeouts` array so calling it from an `#each` block can cause the array iteration to miss elements. Call @block directly instead.
- Change `Reactor#add` to return `false` if the reactor is already shut down instead of invoking the block immediately, so a client-request currently being processed can continue, rather than re-adding to the thread-pool (which may already be shutting down and unable to accept new work).