dbusers.c: Added function for setting read, write and connection timeout values. Set default timeouts for getUsers. Defaults are listed in service.c
gateway.c:shutdown_server is called whenever MaxScale is to be shut down. Added call for service_shutdown to shutdown_server.
service.c:service_alloc: replaced malloc with calloc and removed unnecessary zero/NULL initialization statements as a consequence.
serviceStart: Exit serviceStartPort loop if shutdown flag is set for the service.
serviceStartAll: Exit serviceStart loop if shutdown flag is set for the service.
service.c: Added service_shutdown which sets shutdown flag for each service found in allServices list.
service.h: Added prototype for service_shutdown
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