In theory the pthreads approach for Thread Local Storage should be more
portable.

This also changes some APIs in order to accommodate this change. In
particular ASYNC_init_pool is renamed ASYNC_init_thread and
ASYNC_free_pool is renamed ASYNC_cleanup_thread. Also introduced ASYNC_init
and ASYNC_cleanup.
Reviewed-by: NRich Salz <rsalz@openssl.org>

There are potential deadlock situations that can occur if code executing
within the context of a job aquires a lock, and then pauses the job. This
adds an ability to temporarily block pauses from occuring whilst performing
work and holding a lock.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Clarify that you must only call this after all async jobs have
completed - otherwise you could get memory leaks.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Add some clarifications to the async documentation. Also changed
ASYNC_pause_job() so that it returns success if you are not within the
context of a job. This is so that engines can be used either asynchronously
or synchronously and can treat an error from ASYNC_pause_job() as a real
error.
Reviewed-by: NRich Salz <rsalz@openssl.org>

Reviewed-by: NRich Salz <rsalz@openssl.org>