Stack overflow in ICE on Symbian phone target

[bennylp]

Reduce the stack usage since it caused stack overflow on Symbian phone with default stack size (8KB).

Also reduce the pool usage (avoid to use 4000 bytes blocks) and make the pool size configurable.

[bennylp]

In r1654:

• create a temporary text buffer on pj_ice_sess to avoid using stack buffer for temporary stuffs (such as printing to log, acquiring error message), to reduce stack usage
• don't start periodic check immediately on ICE session creation but schedule an immediate timer for it, to reduce stack usage again.
• equip functions with PJ_CHECK_STACK to track stack usage.
• make pool sizes configurable

[bennylp]

Related to this:

• in r1655: optimize the stack usage in pjsua-lib

[bennylp]

Related to this ticket:

• in r1656: reduce the stack usage of the ICE media transport