Enhance logging so new log files no longer need to wait for the previous log file to be synced

There was a recent bug fix in WT-3000 that means each time WiredTiger switches to a new log file writes need to wait for the previous log file to be flushed before proceeding. That is because otherwise there could be undetected gaps in the log.

An alternative to waiting for the flush would be to write a header record in each new log file indicating the last-known log record location. That would not need to be flushed, and could be used by recovery to verify log file completeness.

The down side of this change is that it introduces a log format change.

It is only of interest to applications that don't configure for synchronous log file writes.