Add support for async jobs in OpenSSL speed

Summary of the changes:

* Move the calls to the crypto operations inside wrapper functions.
  This is required because ASYNC_start_job takes a function as an argument.

* Add new function run_benchmark() that manages the jobs for all the operations.
  In the POSIX case it uses a select() to receive the events from the engine
  and resume the jobs that are paused, while in the WIN case it uses PeekNamedPipe()

* Add new option argument async_jobs to enable and specify the number of async jobs

Example:
  openssl speed -engine dasync -elapsed -async_jobs 32 rsa2048

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>