MINOR: pools: replace DEBUG_POOL_TRACING with runtime POOL_DBG_CALLER

MINOR: pools: replace DEBUG_POOL_TRACING with runtime POOL_DBG_CALLER

This option used to allow to store a pointer to the caller of the last
pool_alloc() or pool_free() at the end of the area. Now that we can
compute the offsets at runtime, let's check it at run time and continue
the code simplification. In __pool_alloc() we now always calculate the
return address (which is quite cheap), and the POOL_DEBUG_TRACE_CALLER()
calls are conditionned on the status of debugging option.