Threadpool Refactor

[COM8]

Refactors cpr::ThreadPool to get rid of all those deadlocks and overly complex code making it at least a bit simpler.

Fixes:

• Thread Pool Deadlock While Task Wait #1040
• Force the end of the thread pool #988

Docs:

• Added cpr::ThreadPool Documentation docs#54

Selecting a Backend

Select the backend when configuring cpr. The builtin backend is the default and adds no dependencies:

cmake -S . -B build -DCPR_THREAD_POOL=BUILTIN
# Use an existing Boost installation (1.70 or newer):
cmake -S . -B build -DCPR_THREAD_POOL=BOOST_ASIO

With CMake FetchContent, set CPR_THREAD_POOL before FetchContent_MakeAvailable(cpr). Other values are rejected at configuration time. cpr does not download Boost; its public headers forward-declare the optional Asio context type. Installed cpr packages built with BOOST_ASIO discover the exact Boost version used to build cpr through their CMake package configuration, for both shared and static libraries.

Backend	Workers	Changing the maximum while running
BUILTIN	Grows on demand up to the maximum; excess idle workers retire after an idle timeout, retaining the minimum.	Excess workers retire as their active tasks finish; queued tasks are preserved.
BOOST_ASIO	Owns a boost::asio::thread_pool with a fixed worker count equal to the maximum. The minimum is retained as configuration.	Waits for active tasks and replaces the executor, preserving queued tasks.
BOOST_ASIO with a caller-owned context	Creates no workers. The maximum limits simultaneous cpr tasks; the minimum is retained as configuration.	Applies the new limit to subsequent task claims; active tasks finish normally.

[portsip]

Hi, I would like to suggest use a third thread pool implementation: https://github.com/bshoshany/thread-pool

[COM8]

Hi, I would like to suggest use a third thread pool implementation: https://github.com/bshoshany/thread-pool

Agree. If I'm unable to fix the macos socket issues, I will most likely switch and experiment with that one. Good point.

[portsip]

The ASIO thread pool is also a good choice.

[roemil]

The ASIO thread pool is also a good choice.

Yes! With ASIO threadpool we could get an awaitable Get/Post and use that within a boost coroutine. That would be nice to have.