Ensures that Relationship commands in upgraded logs have proper first in chain flags

Previously upgraded logs would simply copy over relationships commands, leaving
 default values for the firstInFirstChain and firstInSecondChain fields. This
 could result in pointer chasing errors if these transactions were applied to an
 otherwise properly upgraded store, a scenario that comes up in backup. In
 particular, backup will copy the store and then the upgraded transactions,
 which would update the relevant relationship records to have
 wrong values for those flags, affecting pointer following.
 This commit checks if the currently transferred LogEntry holds
 a RelationshipCommand and if so it sets up the first in chain flags
 properly.
This approach, while improving on the problem, does not completely solve it.
 While the pointers will be interpreted correctly, the first record in a
 relationship chain will still have its previous pointer set to -1, which
 will overwrite the current value that represents the relationship count
 for this chain. This issue affects backups that are created
 immediately after an upgrade from 1.9 or 2.0.