xlog: introduce helpers to check cursor state

It isn't a good practice to access xlog_cursor::state directly, because
it may include implementation-specific states, e.g. we might want to
differentiate CLOSED_AFTER_EOF and CLOSED_WITHOUT_EOF or CLOSED and
NOT_YET_OPENED (see the next patch). Let's introduce convenient helpers
to check if a cursor is open and if it reached EOF and use them outside
xlog.c.

Needed for #2768