log: fix a deadlock caused by conflicting lock order

[BurtonQin]

This patch fixes a possible deadlock caused by conflicting lock order in src/log.rs:

clear_logs	end_record
overlays.write()
	appending.write()
appending.write()//deadlock!
	overlays.write()//deadlock!

The fix is to lift the overlays.write() above appending.write() in end_record().

[arkpar]

Thank you for detecting this. Even though clear_logs is never called in parallel with end_record this might change in the future. Proposed fix is suboptimal though. Calling to_file is expensive and the overlay lock should not be taken at this time.

I've fixed clear_log in 87abacb instead.