Revised Core/Store cooperation w.r.t. SwapDir map slot locks.

Revised Core/Store cooperation w.r.t. SwapDir map slot locks.

When StoreEntry is deleted, we need to release the SwapDir map slot locks it
holds, if any. This is difficult because SwapDir maintains the locks while
Squid Core maintains the entry swap_status. The Core gets swap_status-related
notifications using async calls so it is easy for swap_status to get out of
sync if SwapDir updates the map slot proactively.

The new code no longer releases the slot lock until the associated StoreEntry
is unlinked or gone, even if the slot is known to be unusable and waiting to
be deleted. We also do not rely on swap_status to guess which lock to release;
we use slot state to determine that instead.

Removed rock-specific code from StoreEntry destructor by introducing a general
SwapDir::disconnect(StoreEntry&) API.

Polished StoreEntry initialization.

Polished and fixed Rock::SwapDir debugging.