SHAMapStore with online delete

@JoelKatz @nbougalis 

Here's a more detailed description of this effort:

    SHAMapStore: Online Delete
    Makes rippled configurable to support deletion of all data in its key-value
    store (nodestore) and ledger and transaction SQLite databases based on
    validated ledger sequence number. All records from a specified ledger
    and forward shall remain available in the key-value store and SQLite, and
    all data prior to that specific ledger may be deleted.
    
    Additionally, the administrator may require that an RPC command be
    executed to enable deletion. This is to align data deletion with local
    policy.

Generation is clearly an important concept - it needs more explanation (unless it's elsewhere).

I don't think it's "validity" that's in question, is it?  It's whether something can be garbage collected?

You don't need `(void)` - just say `std::uint32_t getGeneration () const`