manifest: change terminology for storage in context classes
Storage may not be backed by revlogs. So it is wrong to refer
to storage as "revlog" or use abbreviations like "rl."
This commit replaces revlog terminology in manifest context classes
and starts referring to things as "store" or "storage."
Differential Revision: https://phab.mercurial-scm.org/D4389
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors