Move locking responsibility from store.go to the container/image/layer stores

[mtrmac]

Final big part of the preparatory infrastructure work on locking.

Move the conceptual responsibility for locking/loading containerStore, imageStore and layerStore from the caller (store) into the three individual C/I/L objects. And in those objects, centralize it.

I.e. this goes from

func (s *store) method(…) {
    …
    layerStore.Lock()
    defer layerStore.Unlock()
    if err := store.ReloadIfChanged(); err != nil { … }

to

func (s *store) method(…) {
    …
    if err := layerStore.startWriting(); err != nil { … }
    defer layerStore.stopWriting()

that will, finally, allow us to change the design in a single place, startReading / startWriting. E.g. to:

• Do in-process locking correctly (layerStore.Load under layerStore.RLock is racy #1332 )
• Clean up inconsistent state, and write it back to disk, even if the caller is a reader (Crash resiliency: deleting incomplete layers doesn’t reliably happen #1136 )
• Avoid the need to call Locked() or IsReadWrite in the Load method, passing the values from the (now single) caller instead (lockfile.Locked() must not be used to make any decisions #1388 )

---

This should not change observable behavior, apart from the “Remove a redundant rwImageStore.Save() call” commit.

See individual commit messages for details.

(The need to change every lock caller in store by this PR is the primary motivation for preparing #1383 / #1384 , which decreased the number of places to update.)

This is organized by the individual store, changing each one separately; i.e. each conceptual change is repeated three times. If desired, I can instead reorganize this to do same conceptual change in a single commit, across all three stores.

This ends up removing roFileBasedStore / rwFileBasedStore, and declaring the {start,stop}{Reading,Writing} methods in each individual store. If preferred, I could change this PR to keep (or re-introduce) those interfaces again.

[rhatdan]

LGTM
@nalind @flouthoc @giuseppe @vrothberg @saschagrunert PTAL

[giuseppe]

LGTM

[vrothberg]

LGTM