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.

This function splits a DELETE FROM query with a large range into smaller ones, because SQLite requires the lock. The sleep allows other users to get some work done while this job takes a break. We back off essentially a little bit to not hog SQLite. It will only occur 1000 times a second if the SQL delete takes 0 time and happens thousands of times.