manifestcache: protect write with `wlock` instead of `lock`
The `wlock` is taken by both `update` and `commit` type operation. This would
help persisting the cache more aggressively.
An explicit test is introduced. However, we can already see the effect of this
change on earlier test output.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.