This patch makes a significant change to how I/O resources bind to a
reactor. Currently, an I/O resource (TCP, UDP, PollEvented) will bind
itself with a reactor upon creation.
First, some history.
Originally, tokio-core required that I/O resources be explicitly
associated with a reactor upon creation by passing in a `&Handle`. Tokio
reform introduced a default reactor. If I/O resources do not specify a
reactor upon creation, then the default reactor is used.
However, futures tend to favor being lazy. Creating a future should do
no work, instead it is defining a computation to be performed once the
future is executed. Binding an I/O resource with a reactor on creation
goes against this pattern.
This patch fixes this by allowing I/O resources to lazily bind to a
reactor. An explicit `&Handle` can still be used on creation, but if no
reactor is specified, then the default reactor is used. However, this
binding happens during execution time (read / write) and not creation.
