New macros, MReadOption/MWriteOption to act directly on task board attributes
The task board is now initialized during Storage stage

The runtime configuration may be now easily modified during PoolPrepare() stage, through
`MReadOption`and `MWriteOption` macros. Attributes are stored in the master datafile and
broadcasted to all workers right after the PoolPrepare() is finished.
Now, the pool loop fully supports the attributes as runtime configuration

From now on, the recommended way to obtain and manage runtime configuration options is to
use task board attributes, through the `MReadOption` and `MWriteOption` hooks. The `setup`
pointer is deprecated and marked for removal for the full 2.3 release.