support running coroutines on only one thread

[Xudong-Huang]

when user config MAY as following

may::config()::set_workers(1).set_io_workers(0);

MAY should running all the coroutines on a single thread. this would

• help to avoid some tricky sync issue
• allow library to access TLS since coroutines won't scheduled on other thread

This will NOT help the TLS issue. Let coroutines bound to a single thread is still not safe, e.g. coroutine A in the thread set a TLS into value A', and then another coroutine B is scheduled and it set the TLS into value B', after coroutine A rescheduled on this same thread, it will access a dirty value!

[jedisct1]

So, there are no ways to use multiple threads and still use the standard library?

Or is there a way to have multiple May instances, one per thread?

[Xudong-Huang]

As far as users don't access TLS, of course we can have multiple thread scheduling, just like the echo server example.

But if users can't avoid using TLS, this is just a fallback way to let their code running normally.

Running multiple May instances is not considered in this library developing.