A read-only txn isn't serialized by raft, but it uses a fresh
read txn for every mvcc access prior to executing its request ops.
If a write txn modifies the keys matching the read txn's comparisons,
the read txn may return inconsistent results.
To fix, use the same read-only mvcc txn for the duration of the etcd
txn. Probably gets a modest txn speedup as well since there are
fewer read txn allocations.
c87594f27c