Avoid memory pool growing when doing re-Registration

[riza]

When doing re-registration, on some cases we might get different response (e.g: Authentication challenge) which eventually cause the memory pool to grow.

This ticket will use a new pool specific for this case, it will check for the pool size, and recreate it if it's over the limit.

Thanks to Oleg Grazhdan for the patch.

[riza]

In 5336:

Re #1929: Avoid memory pool growing when doing re-Registration.

[ming]

In 5373:

Fixed #1929: Fixed memory leak when using sip_auth_client since it's also used by other components, such as sip_dialog, publishc, and pjsua_im.

Application should use the new API pjsip_auth_clt_deinit() to release the pool.