Merge acts_as_timevector back into acts_as_hypertable metadata

[jonatas]

acts_as_timevector serves as a base to store the segment_by.

Options:

1. Load it directly from compression settings.
2. Explicit it directly in the model independent of compression, just for the sake of leveraging scopes.

Make it explicit to guarantee versions are build accordingly to the explicit code. If compression settings change in the database, we cannot rely on it.

Example of code:

    class PageLoad < ApplicationRecord
      acts_as_hypertable chunk_time_interval: '1 day',
        time_column: 'created_at'

      acts_as_timevector segment_by: 'path'

      scope :stats, -> do 
        select('max(performance), min(performance), avg(performance)')
      end

      continuous_aggregates :stats, timeframes: ['1 hour', '1 day']
    end

vs

    class PageLoad < ApplicationRecord
      acts_as_hypertable chunk_time_interval: '1 day',
        time_column: 'created_at', segment_by: 'path'

      scope :stats, -> do 
        select('max(performance), min(performance), avg(performance)')
      end

      continuous_aggregates :stats, timeframes: ['1 hour', '1 day']
    end