Option to control first re-registration retry interval

[bennylp]

Ticket #1044 adds pjsua_acc_config.reg_retry_interval to control account interval to retry registration in case it failed. However when the registration is disconnected because of transport failure (e.g. TCP transport closure), the initial retry would happen immediately.

This ticket adds new setting, pjsua_acc_config.reg_first_retry_interval, to control the interval of the first retry. The default value is zero, which means registration retry will be done immediately after failure.

Note that in case there are calls active when the TCP transport is disconnected, these calls will be hung up only after the first registration retry fails, hence it is recommended to set pjsua_acc_config.reg_first_retry_interval to low value.

This ticket also adds random +/- 10 seconds to the interval, so that clients wouldn't re-register at the same time thus causing high load to server.