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

@@ -224,3 +224,26 @@ rebalancer_recovery_group.test_no_logs_while_unsuccess_recovery = function(g)
         end)
     end, {hanged_bucket_id})
 end
+
+rebalancer_recovery_group.test_rebalancer_routes_logging = function(g)
+    move_bucket(g.replica_2_a, g.replica_1_a,
+                get_first_storage_bucket_id(g.replica_2_a))
+    move_bucket(g.replica_3_a, g.replica_1_a,
+                get_first_storage_bucket_id(g.replica_3_a))
+    g.replica_1_a:exec(function()
+        ivshard.storage.rebalancer_wakeup()
+    end)
+    t.helpers.retrying({timeout = 10}, function()
+        t.assert(g.replica_1_a:grep_log(
+            'Apply rebalancer routes with 1 workers'))
+    end)
+    t.assert(g.replica_1_a:grep_log('Move 1 bucket'))
+    local route_1_to_2 = string.format('from %s to %s',
+                                       g.replica_1_a:replicaset_uuid(),
+                                       g.replica_2_a:replicaset_uuid())
+    local route_1_to_3 = string.format('from %s to %s',
+                                       g.replica_1_a:replicaset_uuid(),
+                                       g.replica_3_a:replicaset_uuid())
+    t.assert(g.replica_1_a:grep_log(route_1_to_2))
+    t.assert(g.replica_1_a:grep_log(route_1_to_3))
+end

vshard/storage/init.lua

@@ -2882,6 +2882,10 @@ local function rebalancer_service_f(service)
         -- incorrectly.
         assert(next(routes) ~= nil)
         for src_id, src_routes in pairs(routes) do
+            for dest_id, buckets_count in pairs(src_routes) do
+                log.info('Move %s bucket(s) from %s to %s',
+                         buckets_count, src_id, dest_id)
+            end
             service:set_activity('applying routes')
             local rs = M.replicasets[src_id]
             lfiber.testcancel()