Rework portable `blocking_get()` implementation

[camio]

Acceptance criteria

• Investigate the (somewhat) portable implementation of Grand Central Dispatch (GCD) in Swift
• blocking_get() tracks block counts and adds threads to the pool as necessary
• Unittests, etc.

[camio]

Unit tests pass. Will write additional tests and docs. Expect PR at around 6/9.

[sean-parent]

PR #428

[camio]

Completed.