Fabrics: Capability querying, Protocol selection

[jonasohland]

Implement a way for targets and initiators to expose some information about their capabilities. Add a protocol switching layer inside {RC/RDM}Target and {RC/RDM}Initiator.
We need this to support some hardware that has only a limited RDMA featureset, or is limited in the maximum message size.

[jonasohland]

There will need to be some communication between the target and initiator to discover weather or not RMA is supported.

This could happen "in-band" where this negotiation is not visible to this application. That makes it easier for the application and all upper layers to establish a connection. But if an upper layer that wants to establish that connection, it can never be sure if it is getting the performance it was planning with.

The alternative would be an API call to query RMA capability, and the option to request RMA capability when targets and initiators are set-up. That would push the burden to select RMA when it is available to an upper layer, but the upper layer could be sure hat it is getting full RMA performance when it requests it.

There are some other things that need to be negotiated like message size limitations, but I think these should be negotiated in-band, because they don't have that much of an impact on performance.

[jonasohland]

We will work on this on top of #266

[felixpou]

TSC

• @jonasohland will come back with this once PR Fabrics 2/2: Implementation of the fabrics API #266 is done