RDMA/rtrs: client: main functionality

This is main functionality of rtrs-client module, which manages set of
RDMA connections for each rtrs session, does multipathing, load balancing
and failover of RDMA requests.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Danil Kipnis <[email protected]>
Signed-off-by: Jack Wang <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>