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.h

@@ -128,7 +128,6 @@ protected:
     int peer_connect_timeout = DEFAULT_PEER_CONNECT_TIMEOUT;
     int osd_idle_timeout = DEFAULT_OSD_PING_TIMEOUT;
     int osd_ping_timeout = DEFAULT_OSD_PING_TIMEOUT;
-    uint32_t bs_bitmap_granularity = 0;
     int log_level = 0;
     bool use_sync_send_recv = false;
 
@@ -137,6 +136,7 @@ protected:
     std::string rdma_device;
     uint64_t rdma_port_num = 1, rdma_gid_index = 0, rdma_mtu = 0;
     msgr_rdma_context_t *rdma_context = NULL;
+    // FIXME: Allow to configure these options
     uint64_t rdma_max_sge = 128, rdma_max_send = 32, rdma_max_recv = 32;
 #endif
 
@@ -189,6 +189,7 @@ protected:
     void handle_send(int result, osd_client_t *cl);
 
     bool handle_read(int result, osd_client_t *cl);
+    bool handle_read_buffer(osd_client_t *cl, void *curbuf, int remain);
     bool handle_finished_read(osd_client_t *cl);
     void handle_op_hdr(osd_client_t *cl);
     bool handle_reply_hdr(osd_client_t *cl);