Implement RDMA receive with memory copying (send remains zero-copy)

This is the simplest and, as usual, the best implementation :)

100% zero-copy implementation is also possible (see rdma-zerocopy branch),
but it requires to create A LOT of queues (~128 per client) to use QPN as a 'tag'
because of the lack of receive tags and the server may simply run out of queues.
Hardware limit is 262144 on Mellanox ConnectX-4 which amounts to only 2048
'connections' per host. And even with that amount of queues it's still less optimal
than the non-zerocopy one.

In fact, newest hardware like Mellanox ConnectX-5 does have Tag Matching
support, but it's still unsuitable for us because it doesn't support scatter/gather
(tm_caps.max_sge=1).

README.md

@@ -43,6 +43,7 @@ breaking changes in the future. However, the following is implemented:
 - Inode metadata storage in etcd
 - Snapshots and copy-on-write image clones
 - Write throttling to smooth random write workloads in SSD+HDD configurations
+- RDMA/RoCEv2 support via libibverbs
 
 ## Roadmap
 
@@ -54,7 +55,7 @@ breaking changes in the future. However, the following is implemented:
 - Scrubbing without checksums (verification of replicas)
 - Checksums
 - Tiered storage
-- RDMA and NVDIMM support
+- NVDIMM support
 - Web GUI
 - Compression (possibly)
 - Read caching using system page cache (possibly)