Remove bogus "found KEY_PRIVATE" WARNINGs (#862)

Remove bogus "found KEY_PRIVATE" WARNINGs (#862)

... triggered by private bumped StoreEntry unlock()ed in ~ServerBump().

The WARNINGs were added long time ago (commit fc8b9fc) because, AFAICT,
earlier Store code expected StoreEntry owners to release() uncachable
entries, including KEY_PRIVATE ones, right before unlocking them.
However, there is no compile-time enforcement of that expectation, and
unlocking code does not always know whether the entry is cachable (as
ServerBump constructor/destructor RAII code illustrates).

This change stops tying release and unlocking decisions/actions together
but makes sure that idle KEY_PRIVATE entries are still released (because
we do not want to index unneeded/unusable Store entries).