There are four possible states for the WT_PAGE.dsk memory: the WT_PAGE
disk-image is mapped into memory (unmap on page discard), is in allocated
memory (free on page discard), is in memory belonging to a caller (ignore on
page discard), or isn't set at all (ignore on page discard).

We've been trying to figure out which based on the state of WT_PAGE.dsk and
a single flag, and that's not enough.  Use two explicit flags, one for
mapped memory and one for allocated memory, and neither flag is set for the
latter two states.