When updating ShmemVariableCache from a checkpoint record, be sure to set

all the values derived from oldestXid, not just that field.  Brain fade in
one of my patches associated with flat file removal, exposed by a report
from Fujii Masao.

With this change, xidVacLimit should always be valid, so remove a couple of
bits of complexity associated with the previous assumption that sometimes
it wouldn't get set right away.