[xAttrs] after failover and recovery, some virtual xAttrs lose data

2 node cluster with each node having kv,index,query services(fresh installs). You can reproduce this by loading `travel-sample` then doing 

1. graceful failover one of the nodes
2. full recover the failover and rebalance it back in
3. run SELECT meta().xattrs.`$document` FROM `travel-sample` where meta().xattrs.`$document`.last_modified is missing

I don't know if this is a query issue or if its a rebalance issue or a failover issue, so I am assigning to query.

I noticed an issue when I was fixing some tests. basically if you run this test, since this test has been failing due to testware I am not sure which build last passed:

./testrunner -i toybuild.ini -p doc-per-day=1,nodes_init=2 -t tuqquery.tuq_xattr.QueryXattrTests.test_hard_failover_and_full_recovery_and_gsi_rebalance

You will see that some virtualXAttrs do not contain certain data after the failover and full recovery is performed. Seven documents appear (out of 2016) without the last_modified field. 

SELECT meta().xattrs.`$document` FROM default where meta().xattrs.`$document`.last_modified is missing

[
{
"$document": {
"CAS": "0x152f49d79a100000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x000000000000000b",
"value_bytes": 408,
"value_crc32c": "0x3ccb766e",
"vbucket_uuid": "0x00002ce8939be66d"
}
},
{
"$document": {
"CAS": "0x152f49d7f2cf0000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x000000000000000f",
"value_bytes": 406,
"value_crc32c": "0xf14c7470",
"vbucket_uuid": "0x000006fb735ac372"
}
},
{
"$document": {
"CAS": "0x152f49d81aed0000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x000000000000000f",
"value_bytes": 406,
"value_crc32c": "0xc716c962",
"vbucket_uuid": "0x000036c85107a55d"
}
},
{
"$document": {
"CAS": "0x152f49d8404f0000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x000000000000000f",
"value_bytes": 417,
"value_crc32c": "0xfde4bb78",
"vbucket_uuid": "0x00001a8187e18b6b"
}
},
{
"$document": {
"CAS": "0x152f49d8b01d0000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x0000000000000012",
"value_bytes": 408,
"value_crc32c": "0x34a69063",
"vbucket_uuid": "0x000006fb735ac372"
}
},
{
"$document": {
"CAS": "0x152f49d8d5a70000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x0000000000000012",
"value_bytes": 415,
"value_crc32c": "0xbadcd6f3",
"vbucket_uuid": "0x000036c85107a55d"
}
},
{
"$document": {
"CAS": "0x152f49d8df2d0000",
"datatype": [
"json",
"xattr"
],
"deleted": false,
"exptime": 0,
"flags": 4042322160,
"seqno": "0x0000000000000012",
"value_bytes": 405,
"value_crc32c": "0xa0460c7b",
"vbucket_uuid": "0x00001a8187e18b6b"
}
}
]