fix rdma support

[domsj]

#498 changed the default lwt engine to libev.
For rdma environments users will want to override this lwt engine by setting the ALBA_LWT_ENGINE env variable to rselect.
However, there is still an issue in that case: the process will not be able to handle more than 1024 file descriptors.
(Also commands such as asd-multistatistics will currently be really slow - apparently setting up new rdma connections takes a long time?)

So for full proper rdma support I think the following needs to happen:

• fix the 1024 fd limit (in ordma package)
• fix slowness of creating new connections (mellanox should pick this up?)
• change default lwt engine to one that support rdma and tcp/ip that works for more than 1024 fds

[toolslive]

There's also this:
ofiwg/librdmacm#1
also seen by Alba's sister project:
/volumedriver/#245