Let's use "sync(reason=start)" instead of dedicated "start" event for
uniformity. Periodic syncs are now "sync(reason=periodic)" and after
logrotation support there will be also "pre-logrotate" and
"post-logrotate" reasons. Emit "sync(reason=stop)" at xlog shutdown for
uniformity and to make it more clear from looking at just enb.xlog about
what is xlog state at the end.

Stop requiring "start" to be present in the header - we will soon rework
xlog reader to look around for nearby sync automatically so that reading
could be started from any position in the stream.