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=bugfx

Author: mrForza

test/storage-luatest/storage_1_1_1_test.lua

@@ -174,3 +174,28 @@ 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)
+    local moved_bucket_from_2 = vtest.storage_first_bucket(g.replica_2_a)
+    local moved_bucket_from_3 = vtest.storage_first_bucket(g.replica_3_a)
+    move_bucket(g.replica_2_a, g.replica_1_a, moved_bucket_from_2)
+    move_bucket(g.replica_3_a, g.replica_1_a, moved_bucket_from_3)
+    g.replica_1_a:exec(function()
+        ivshard.storage.rebalancer_wakeup()
+    end)
+    t.helpers.retrying({timeout = 60}, 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))
+    move_bucket(g.replica_1_a, g.replica_2_a, moved_bucket_from_2)
+    move_bucket(g.replica_1_a, g.replica_3_a, moved_bucket_from_3)
+end

vshard/storage/init.lua

@@ -1,6 +1,7 @@
 local log = require('log')
 local luri = require('uri')
 local lfiber = require('fiber')
+local json_encode = require('json').encode
 local lmsgpack = require('msgpack')
 local netbox = require('net.box') -- for net.box:self()
 local trigger = require('internal.trigger')
@@ -2882,6 +2883,13 @@ local function rebalancer_service_f(service)
         -- incorrectly.
         assert(next(routes) ~= nil)
         for src_id, src_routes in pairs(routes) do
+            local routes_info = {}
+            for dest_id, buckets_count in pairs(src_routes) do
+                table.insert(routes_info,
+                             string.format('Move %s bucket(s) from %s to %s',
+                                           buckets_count, src_id, dest_id))
+            end
+            log.info(json_encode(routes_info))
             service:set_activity('applying routes')
             local rs = M.replicasets[src_id]
             lfiber.testcancel()