Correction to check-ins (6246) and (6247): The backup object might not

Correction to check-ins (6246) and (6247):  The backup object might not
hold a valid destination connection pointer.  Also, do not reset the
page cache when establishing a read-lock while there is a persistent
or truncated journal, only if there is a journal that really needs to
rollback.  Otherwise backups always reset whenever the source database
file is read. (CVS 6248)

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