Use parasitic context by default in Classic pipe pattern

[leviramsey]

Inspired by a side discussion in #31906, makes the default ExecutionContext for the Classic pipe pattern for Scala futures the parasitic (aka "same thread") context.

The Typed context.pipeToSelf counterpart only uses the parasitic context (the Classic pipe pattern is still required for piping a future to an actor that's not self). For Java futures, the ExecutionContext setting has no effect, but the use of whenComplete in both Classic and Typed has effectively the same semantics.

A local run of MiMa reported no binary issues (a little surprisingly...). Since existing source code is putting a context in the implicit scope (most likely the context/system dispatcher), new/edited code is required in order to take advantage of this, but this does remove a reason to have an implicit ExecutionContext in an actor.

[He-Pin]

I was wondering if it was because a BlockingQueue may be used for a mailbox?

[patriknw]

I was wondering if it was because a BlockingQueue may be used for a mailbox?

That can be the original reason. I don't think we should risk introducing any problem by changing this.

[leviramsey]

If BlockingQueue is the reason, then that would argue for using whenCompleteAsync on the CompletionStage variant (at least if the EC is an ExecutionContextExecutor), would it not?