Hybrid `bsp_put' payload buffer

[Tombana]

We want to implement a system where the payload of `bsp_put' is stored in different ways:

• the first ~ 32 bytes (8 floats or integers) that are transferred using `bsp_put' are stored in on-core memory
• the next ~ 1024 bytes are stored in external memory, separate buffer for each core
• all remaining bytes are stored in external memory, single buffer for all cores. Needs mutex protection which makes it really slow