Move and rename several classes to organise them by function.

[clemahieu]

Moved in to nano::store:

• nano::store::make_store
• nano::store::write_database_queue

Moved in to nano::voting:

• nano::local_vote_history -> nano::voting::history
• nano::vote_cache -> nano::voting::cache
• nano::vote_processor -> nano::voting::processor
• nano::vote_generator -> nano::voting::generator
• nano::request_aggregator -> nano::voting::aggregator
• nano::vote_spacing -> nano::voting::spacing