re-add lock journal before reading journal files

re-add lock journal before reading journal files

This reverts commit 2e6e9876e3fc420a22eee82eedec54ac3f678878.

This is gonna be needed after all.. The append will only be atomic if
the journal is locked, because the file being appended will have to be
moved out of the way to avoid an old version of git-annex seeing an
incomplete write to it. When git-annex finds that the file is not in the
journal, and checks the append location, locking will be needed to avoid
a race causing it to miss it in the append location too due to it being
moved back to the journal.