Added API to retrieve number of transactions and dialogs (thanks Sergey Bakulin)

[bennylp]

Some application needs to query the number of transactions and dialogs currently active in the stack.

[bennylp]

Added these two functions in r1594:

In sip_transaction.h:

/**
 * Retrieve the current number of transactions currently registered
 * in the hash table.
 *
 * @return        Number of transactions.
 */
PJ_DECL(unsigned) pjsip_tsx_layer_get_tsx_count(void);

In sip_ua_layer.h:

/**
 * Retrieve the current number of dialog-set currently registered
 * in the hash table. Note that dialog-set is different than dialog
 * when the request forks. In this case, all dialogs created from
 * the original request will belong to the same dialog set. When
 * no forking occurs, the number of dialog sets will be equal to
 * the number of dialogs.
 *
 * @return        Number of dialog sets.
 */
PJ_DECL(pj_uint32_t) pjsip_ua_get_dlg_set_count(void);