CAS and expiry changed after online upgrade

In xdcr online upgrade test, we noticed that the CAS and expiry of documents changed after online upgrade.

1. Before upgrade, cluster consisted of a single node, A, with version 2.5.0-1059-rel. Document was as follows:

Doc seq: 6
id: loadOne175
rev: 2
content_meta: 131
size (on disk): 51
cas: 1432797353837527040, expiry: 1432797852, flags: 0
size: 512
data: (snappy) loadOne-175aaaaaaaaa....

2. A second node, B, with version sherlock-3039 was rebalanced into the cluster. The document on node B was as follows:

Doc seq: 4
id: loadOne175
rev: 3
content_meta: 131
size (on disk): 65
cas: 1432797821493444608, expiry: 1432798320, flags: 0, datatype: 0, conflict_resolution_mode: 0
size: 512
data: (snappy) loadOne-175aaaaaaaaaaaaa...

Note that the CAS and expiry of the document had changed.

3. Rebalanced out node A. Cluster ended up with a single node B with version sherlock-3039. The document on node B remained the same as that in 2.

In our test, we are using the following test script to control the upgrade.

./testrunner -i upgrade.ini -p upgrade_version=4.0.0-3039,initial_vbuckets=1024 -t "xdcr.upgradeXDCR.UpgradeTests.online_cluster_upgrade,initial_version=2.5.0-1059-rel,bucket_topology=default:1>2;standard_bucket_1:1<2;sasl_bucket_1:1><2,expires=500,post-upgrade-actions=src-rebalancein;dest-rebalanceout"

The upgrade.ini file is attached.

The test is pretty complex with multiple nodes in a cluster and upgrade being done on both source and target clusters. The test data shown above was produced by a simplified version of the test using only one node in a cluster. The issue should be reproducible in a simplified manual test.