Add ASYNC_block_pause and ASYNC_unblock_pause

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: Rich Salz <rsalz@openssl.org>

include/openssl/async.h

@@ -78,6 +78,8 @@ int ASYNC_get_wait_fd(ASYNC_JOB *job);
 ASYNC_JOB *ASYNC_get_current_job(void);
 void ASYNC_wake(ASYNC_JOB *job);
 void ASYNC_clear_wake(ASYNC_JOB *job);
+void ASYNC_block_pause(void);
+void ASYNC_unblock_pause(void);
 
 /* BEGIN ERROR CODES */
 /*