Remove BackgroundAction after ImageDecoder stopped using it. Drop its
dedicated test target and sources from LibThreading.
Replace the LibCore stream tests' remaining usage with explicit Thread
objects so the tree no longer depends on the removed API.
This commit splits out synchronization primitives from LibThreading into
LibSync. This is because LibThreading depends on LibCore, while LibCore
needs the synchronization primitives from LibThreading. This worked
while they were header only, but when I tried to add an implementation
file it ran into the circular dependency. To abstract away the pthread
implementation using cpp files is necessary so the synchronization
primitives were moved to a separate library.
Add a simple thread pool with a fixed number of worker threads and a
shared work queue. The pool is accessed via ThreadPool::the() which
lazily creates a singleton with 4 worker threads.
submit() enqueues a work item and signals a condvar. Worker threads
loop waiting on the condvar, picking up and executing work items.
Worker threads use 8 MiB stacks to match the main thread, since
the JS parser can build deep call stacks during off-thread parsing.
