Revisit priorities being 32 bits

[abouteiller]

Description

Priorities can overflow for common values of MT/NT (as seen in ICLDisco/dplasma#56). It is quite hard to change the priorities in dplasma to fit in 32 bits, and it may have essentially 0 cost to increase priority field to 64 bit in parsec.

Describe the solution you'd like

increase priority field to 64 bit and maintain the task structure under a cache line.
Maintaining the size under cache line may not be possible, we would have to decide then if the presumably small cost is worth the risk of breaking user code.