LP#1339190: Let workers hang around for a bit for higher-rate clients

LP#1339190: Let workers hang around for a bit for higher-rate clients

For some clients, such as AMH (sorters), the per-message connections
cause too much latency.  So, instead, we'll let their backends hang
around for a while.  This is controlled by a new attribute on the
server-params element, worker-keepalive, as a peer to the personality
value.

This is measured in seconds and the default is 5. A value of 0 here
will disable this feature altogether.

Signed-off-by: Mike Rylander <mrylander@gmail.com>