#2 add tunable options

[Mark Heily]

The current libpwq implementation is oriented towards client programs. We should add tuning options suitable for server programs. The following tuning has been reported to improve behavior in a server environment:

• Increase the number of initial and minimum threads to 100.
• Make the manager thread wake up every 10ms to be more responsive to sudden bursts of traffic.
• Increase WORKER_IDLE_SECONDS_THRESHOLD to 10 minutes, so we don't tear down threads aggressively.

I propose we create a public data structure named pwq_config that contains all of the tunable options for the library. Clients would be allowed to manipulate these options directly. Example:

   pwq_config.minimum_threads = 100;
   pwq_config.initial_threads = 100;
   pwq_config.worker_idle_seconds_threshold = 600;
   pwq_config.manager_wake_interval = 10;

In addition to exposing all of the tuning options, it would be nice to have the concept of a "profile" so you could specify the type of environment and have the library set the corresponding tuning options. . There would be function calls to allow you to set the desired profile. Example:

    typedef enum {
      PWQ_PROFILE_DEFAULT,
      PWQ_PROFILE_CLIENT,
      PWQ_PROFILE_SERVER,
      PWQ_PROFILE_EMBEDDED,
    } pwq_profile_types;

    pthread_workqueue_set_profile(PWQ_PROFILE_SERVER);