Add autofilling for `OracleSet.LastUpdateTime`

[mvadari]

rippled time is weird, and it's a bit hard to determine what the proper LastUpdateTime should be. Adding support to autofill it could be useful.

There is some code that does this in tests, though it could probably be improved to use a more current time rather than the last ledger's close time.

[ckeshava]

I tried to use the current time in the tests. However, this intermittently (not consistently reproducible) results in a temMALFORMED error. Depending on when the ledger is closed, the "current time" could be in the future. This causes rippled to throw an error.

Love to hear other ideas