[stellar/stellar-core] change «manual close» to use `VIRTUAL_TIME` (#1760)

manual close mode is used when testing, and can be used to replay transactions or debug issues;

there are a too many «gotcha’s» that come with the way manual close works right now (for example, if the caller attempts to close too fast, some failures occur, or certain timeouts trigger).

Best option seems to be:
* to use `VIRTUAL_TIME`
* move the clock forward by 5 seconds when closing
* allow to set the clock to an arbitrary date (optional parameter to `manualclose`, probably in the same unit than the `close time` used in the ledger header).
* This allows to replay transaction sets exactly the same way than found on a live network for example.