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).

src/messenger.cpp

@@ -139,9 +139,6 @@ void osd_messenger_t::parse_config(const json11::Json & config)
     this->rdma_gid_index = (uint8_t)config["rdma_gid_index"].uint64_value();
     this->rdma_mtu = (uint32_t)config["rdma_mtu"].uint64_value();
 #endif
-    this->bs_bitmap_granularity = strtoull(config["bitmap_granularity"].string_value().c_str(), NULL, 10);
-    if (!this->bs_bitmap_granularity)
-        this->bs_bitmap_granularity = DEFAULT_BITMAP_GRANULARITY;
     this->use_sync_send_recv = config["use_sync_send_recv"].bool_value() ||
         config["use_sync_send_recv"].uint64_value();
     this->peer_connect_interval = config["peer_connect_interval"].uint64_value();