In order to determine whether we need to rebootstrap the instance on
startup, we need to know its vclock. To find it out, we are planning to
scan the last xlog file before proceeding to local recovery, but this
means in case rebootstrap is not required we will scan the last xlog
twice, which is sub-optimal. To speed up this procedure, let's create a
new empty xlog before shutting down the server and reopen it after
restart.

Needed for #461