StorageLoader: replace table configuration property with schema

[alexanderdean]

The problem with the current approach is that it suggests that:

:targets:
  - :name:     All time
    :type:     redshift
    :host:     redshift.acme.com
    :database: snowplow
    :table:    atomic.events
    ...
  - :name:     Rolling 24h
    :type:     redshift
    :host:     redshift.acme.com
    :database: snowplow
    :table:    atomic.events_last_1d

is safe. In fact, the above is highly unsafe - because both loads would populate the same child tables in atomic.

For this reason, we should remove the end user control over the events table (which is currently the only table the user can rename) and replace the outdated table option with a schema option (correctly implying that each distinct Snowplow load needs to be sandboxed into its own schema).