thread_pool with default constructor is broken

[sneves]

thread_pool in the most recent Asio version, as seen on Boost 1.87.0, is broken. Example code:

boost::asio::thread_pool pool {};
boost::asio::post(pool, [] {
  fmt::println("hello");
});
pool.join();

This will, more often than not, simply not print anything. The root cause here is found in commit 480e88b where thread_pool::thread_pool() was not updated to also set joinable_(true).