Add API to limit retransmission count for outgoing requests in transaction layer

[nanangizz]

This PR introduces a new API pjsip_tsx_set_max_retransmit_count(), docs:

 * Set maximum retransmission count in transaction layer for outgoing
 * requests. When retransmission counter reaches the specified number,
 * the transaction will be considered timeout. This will affect any
 * ongoing transactions immediately.
 *
 * By default (or when this is set to -1), retransmission number will
 * not cause a transaction to be timeout, only the timer settings will
 * cause transaction timeout. Also, this will not override the timer
 * settings, i.e: if the timeout timer occurs before the maximum
 * retransmission is reached, the transaction will still gets timeout.
 *
 * When this is set to zero or possitive number P, a transaction timeout
 * will occur right before the retransmission number (P+1). For example,
 * if this is set to 1 there will be two transmissions: the initial
 * transmission and one retransmission, before the transaction gets timeout.