btl/am-rdma: Add per-btl storage and configuration

Add per-btl storage for the am-rdma interface, allowing the interface
to make one-time decisions about whether or not to use the underlying
put/get interfaces.  This is a necessary step in allowing users of the
am-rdma interface to require different semantics (completion,
registration, etc.) than what is natively provided by the BTL.

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

Author: bwbarrett