Add BackpressureMailbox, a Mailbox implementation backed by
queue.BackpressureQueue that consults a queue.DropCheckFunc on every
Send/TrySend to enable RED-style load shedding before the mailbox is
full.
Add MailboxFactory type and ActorOption functional options
(WithMailboxFactory, WithMailboxSize) so callers can inject custom
mailbox implementations when spawning actors via RegisterWithSystem
or ServiceKey.Spawn.
In this commit, we add the actual Actor implementation. We define a
series of types and interfaces, that in concert, describe our actor. An
actor has some ID, a reference (used to send messages to it), and also a
set of defined messages that it'll accept.
An actor can be implemented using a simple function if it's stateless.
Otherwise, a struct can implement the Receive method, and handle its
internal message passing and state that way.
In this commit, we add two new fundamental data structures: Future[T]
and Promise[T].
A future is a response that might be ready at some point in the future.
This is already a common pattern in Go, we just make a type safe wrapper
around the typical operations: block w/ a timeout, add a call back for
execution, pipeline the response to a new future.
A promise is an intent to complete a future. Typically the caller
receives the future, and the callee is able to complete the future using
a promise.
