Ticket 50389 - ns-slapd craches while two threads are polling the same connection
    
    Bug Description:
    	nspr IO is not multi-threaded safe.
    	389-ds should not be in a situation where several threads are polling
    	a same connection at the same time.
    	The scenario is a worker send back an operation result at the same time
    	another worker wants to read an incoming request.
    
    Fix Description:
    	The fix consist in synchonizing polling with c_pdumutex.
    
    	The thread that sends data (flush_ber) hold c_pdumutex.
    
    	The thread that reads the data does a non blocking read. It then
    	enforce ioblocktimeout with iteration of poll.
    	The reading thread must hold c_pdumutex during poll to synchronize
    	with the reader thread.
    	The reading thread must poll with a small timeout
    	(CONN_TURBO_TIMEOUT_INTERVAL). In order to not block
    	the thread that send back data, the fix reduces the delay to 0.1s.
    
    https://pagure.io/389-ds-base/issue/50389
    
    Reviewed by: Mark Reynolds, Matus Honek, William Brown
    
    Platforms tested: F28
    
    Flag Day: no
    
    Doc impact: no
    
    (cherry picked from commit 2886ba77f664e4734a7ddfe4146f229caca49ce4)