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/20200511135131.27580-7-danil.kipnis@cloud.ionos.com
Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>