rgw: set num_shards on 'radosgw-admin data sync init'

[cbodley]

Fixes: http://tracker.ceph.com/issues/22083 (see also #18852)

adds ceph-dencoder support for rgw sync status types, used to validate the fix

without the fix:

$ bin/radosgw-admin -c run/c2/ceph.conf data sync init --source-zone a1
$ bin/rados -c run/c2/ceph.conf -p a2.rgw.log get datalog.sync-status.c00ae4fb-6819-4efc-815d-1e30dfcd24df data.sync-status
$ bin/ceph-dencoder type rgw_data_sync_info import data.sync-status decode dump_json
{
    "status": "building-full-sync-maps",
    "num_shards": 0,
    "instance_id": 988322046502566107
}

with the fix:

$ bin/radosgw-admin -c run/c2/ceph.conf data sync init --source-zone a1
$ bin/rados -c run/c2/ceph.conf -p a2.rgw.log get datalog.sync-status.1b6d8d3f-fbdb-402a-a92b-f19e5b474c1e data.sync-status
$ bin/ceph-dencoder type rgw_data_sync_info import data.sync-status decode dump_json                                                             
{
    "status": "building-full-sync-maps",
    "num_shards": 128,
    "instance_id": 14612016681866375495
}

[yuriw]

wip-yuri2-testing-2017-11-14-1622

[cbodley]

jenkins test this please

[cbodley]

hmm, dencoder tests failing on the new types

edit: generate_test_instances() was using real_clock::now() to set timestamps, so the output of consecutive tests was different