Add a store handle that shares one mapping

Opening a draft by path opened the directory and the file, mapped the
whole file, read the head, and undid all of it on close: nine system
calls for every GET. pure_store_open keeps the store open for one owner
thread; drafts opened through it share a reference-counted mapping of the
newest version, and opening one only checks whether the file changed (its
identity and its head: two system calls). A commit of its drafts maps the
new version once and moves them all onto it. A compaction by another
writer is noticed and followed, while drafts of the old file keep reading
it and relocate their extents when they commit.

pure_store_data gives modules the same through a handle.

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