xlog: store prev vclock in xlog header

This patch adds a new key to xlog header, PrevVclock, which contains the
vclock of the previous xlog file in the xlog directory. It is set by
xdir_create_xlog() to the last vclock in xdir::index. The new key is
only present in XLOG files (it doesn't make sense for SNAP or VYLOG
anyway). It will be used to make the check for xlog gaps more thorough.