Split Rock-only Rock::DirMap into Rock::DirMap and reusable Ipc pieces

Split Rock-only Rock::DirMap into Rock::DirMap and reusable Ipc pieces
which a shared memory cache implementation can use:

Ipc::StoreMap is responsible for maintaining a collection of lockable slots,
each with readable/writeable/free state and a "waiting to be free" flag. Kids
of this class can add more metadata (in parallel structures using the same
index as primary slots). I tried extending the slots themselves, but that
turned out to be more complex/messy.

Ipc::ReadWriteLock is a basic multiple readers, single writer lock.  Its
earlier implementation inside Rock::DirMap mixed slot locking and slot
state/flags. That simplified the caller code a little, but the current simpler
class is easier to understand and reuse.

Rock::DirMap now just adds Rock::DbCellHeader metadata to Ipc::StoreMap slots.

Simplified mapping API by reducing the number of similar-but-different
methods. For example, instead of putAt, the caller can use an
openForWriting/closeForWriting pair. This helps with moving custom metadata
manipulations outside of the reusable Ipc::StoreMap.

It would be possible to split Ipc::StoreMap further by moving Store-specific
bits outside of its slots. Currently, there is no need for that though.