ITS#2389 - stop reading requests on a conn if it has more than

ITS#2389 - stop reading requests on a conn if it has more than
connection_pool_max/2 operations already executing. May want to tune
this to a higher value, e.g. connection_pool_max - XX, etc. Fix in
result.c throttles the connection too late, all threads are already
consumed by the time the send buffer blocks.