Make various event/presence subscription time settings configurable (thanks Johan Lantz for the suggestion)

[bennylp]

Export the following internal macros so that they are overridable from config_site.h:

• PJSIP_PRES_DEFAULT_EXPIRES (was PRES_DEFAULT_EXPIRES):
  Specify the default expiration time for presence event subscription, for both client and server subscription. For client subscription, application can override this by specifying positive non-zero value in "expires" parameter when calling #pjsip_pres_initiate(). For server subscription, we would take the expiration value from the Expires header sent by client in the SUBSCRIBE request if the header exists and its value is less than this setting, otherwise this setting will be used.
• PJSIP_EVSUB_TIME_UAC_REFRESH (was TIME_UAC_REFRESH):
  Specify the time (in seconds) to send SUBSCRIBE to refresh client subscription before the actual interval expires.
• PJSIP_EVSUB_TIME_UAC_TERMINATE (was TIME_UAC_TERMINATE):
  Specify the time (in seconds) to wait for the final NOTIFY from the server after client has sent un-SUBSCRIBE request.
• PJSIP_EVSUB_TIME_UAC_WAIT_NOTIFY (was TIME_UAC_WAIT_NOTIFY):
  Specify the time (in seconds) for client subscription to wait for another NOTIFY from the server, if it has rejected the last NOTIFY with non-2xx final response (such as 401). If further NOTIFY is not received within this period, the client will unsubscribe.
• PJSIP_PUBLISHC_DELAY_BEFORE_REFRESH:
  Specify the time (in seconds) to send PUBLISH to refresh client publication before the actual interval expires.

[bennylp]

Done in r2730

[bennylp]

In r2754:

• added PJSIP_PUBLISHC_DELAY_BEFORE_REFRESH:
  Specify the time (in seconds) to send PUBLISH to refresh client publication before the actual interval expires.