rebalancer: add logging of routes

This patch adds rebalancer routes' logging. The log file now
includes information about the source storage, the number of
buckets, and the destination storage where the buckets will
be moved.

Part of #212

NO_DOC=bugfix

Author: mrForza

test/storage-luatest/storage_1_1_1_test.lua

@@ -195,3 +195,28 @@ rebalancer_recovery_group.test_no_logs_while_unsuccess_recovery = function(g)
     assert_bucket_is_transferred(g.replica_2_a, g.replica_1_a,
                                  hanged_bucket_id_2)
 end
+
+rebalancer_recovery_group.test_rebalancer_routes_logging = function(g)
+    local moved_bucket_from_2 = vtest.storage_first_bucket(g.replica_2_a)
+    start_bucket_move(g.replica_2_a, g.replica_1_a, moved_bucket_from_2)
+    local moved_bucket_from_3 = vtest.storage_first_bucket(g.replica_3_a)
+    start_bucket_move(g.replica_3_a, g.replica_1_a, moved_bucket_from_3)
+    t.helpers.retrying({timeout = 60}, function()
+        g.replica_1_a:exec(function()
+            ivshard.storage.rebalancer_wakeup()
+        end)
+        t.assert(g.replica_1_a:grep_log(
+            'Apply rebalancer routes with 1 workers'))
+        end)
+    local rebalancer_routes_msg = string.format("{\"%s\":{\"%s\":1,\"%s\":1}}",
+                                             g.replica_1_a:replicaset_uuid(),
+                                             g.replica_3_a:replicaset_uuid(),
+                                             g.replica_2_a:replicaset_uuid())
+    t.assert(g.replica_1_a:grep_log(rebalancer_routes_msg))
+    start_bucket_move(g.replica_1_a, g.replica_2_a, moved_bucket_from_2)
+    start_bucket_move(g.replica_1_a, g.replica_3_a, moved_bucket_from_3)
+    assert_bucket_is_transferred(g.replica_1_a, g.replica_2_a,
+                                 moved_bucket_from_2)
+    assert_bucket_is_transferred(g.replica_1_a, g.replica_3_a,
+                                 moved_bucket_from_3)
+end

vshard/storage/init.lua

@@ -2885,6 +2885,7 @@ local function rebalancer_service_f(service)
         -- then max_disbalance would have been calculated
         -- incorrectly.
         assert(next(routes) ~= nil)
+        log.info(json_encode(routes))
         for src_id, src_routes in pairs(routes) do
             service:set_activity('applying routes')
             local rs = M.replicasets[src_id]