Provide a way to configure Runtime threading behavior

[clintfred]

As a ironoxide user
I want to be able to configure the threading behavior of the library
so that I can tune the library for my use case

AC:

• a config object can be provided on initialize, giving the caller appropriate knobs for controlling threading behavior
• threading behavior is verified to match configuration settings
• Default instance is provided for callers who don't care to explicitly set a config

[clintfred]

Some work has been done here, but configuration of the shared Runtime for BlockingIronOxide isn't working as expected. See comments blocking_ops.rs for more details.

[coltfred]

@clintfred Should that link to a branch somewhere? Which comments are you talking about?

[clintfred]

Updated comment above with a link.