When a PG splits the log entries are divided between the two PGs,
this can result in PWLC refering to log entries in the other PG.
Rollback PWLC after the split so it is not further advanced that
the most recently completed log entry.
Non-primary shards can be missing log entries and may rollback
PWLC too far because of this, however this does not matter
because a split occurs at the start of a peering cycle and these
shards will be updated with the correct PWLC from the primary
shard later in the peering cycle when they are activated.
Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>