async/rdma: initialize worker in RDMAStack::create_worker()

async/rdma: initialize worker in RDMAStack::create_worker()

in ff65c800b3e1f3f7e3989223b9bde4cbbaf5c076, we moved create_worker()
call out of the constructor to avoid calling virtual functions in
constructor. but this created a regression where RDMAStack's constructor
tries to reference its workers not yet created.

in this change, the workers are initialized right after they are
created.

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 103ec86cfab3e6807602b128346212bc8eb0d88d)