Add new WPDB Storage Adapter

Add new WPDB Storage Adapter, instruct Stream to use it
Provide a filtering mechanism to alter storage engine used
Alter erase_stream_records() to use db::delete()
Use db::delete() instead of manual queries
Abstract main db class
PHPCS fix
Change query::fields=with-meta to fields=meta
Abstract WP_Stream_ListTable::get_found_rows()
Change log() to use new DB var
Abstract wp_stream_query
Abstract wp_stream_existing_records
CS fix
Abstract get_ips()
Remove db.php in favor of db/base.php
Move filters/actions and common tasks to base db class
Remove constructor call
Misc fixes
Correct db var reference
Provide defaults for new entries, should be the same as default value for columns in MySQL
Item ID is not always an integer, should pursue this even more
WP_DIE instead of exception, better UI and for UnitTests