Introduce tail log entry to the end of a log file

[siying]

Right now, after a machine crash, we don't know whether a log file is complete. For example, we may contain 4.log 5.log but 4.log is partial, and has several entries missing in the end. By adding a tail entry to the end of each closed log file, we can detect this case, so that we know we shouldn't continue relaying 5.log

[rozuur]

Can I work on this?

[ajkr]

quick question: don't we know 4.log is complete if its latest seqnum is one less than 5.log's earliest seqnum?

[siying]

@ajkr not the case if people ever write with WriteOptions.disableWAL=true.

[siying]

@rozuur you are welcome to take it.