Adds the ability to retry for the pool of connection if specified in makara config

[vishnun]

This is an attempt to do graceful failover by re-trying at the lowest level. (This isn't ready to be merged in master for makara but we'll use this branch)

Example config:

development:
  database: speckel_development
  adapter: postgresql_makara
  host: localhost
  username: speckel
  password: 23432
  pool: 20
  makara:
    id: 'development-app-postgres'
    disable_blacklist: true
    master_ttl: 0.3
    sticky: true
    retry_exceptions:
      - name: ActiveRecord::StatementInvalid
        retry_count: 5
        time_between_retries_in_seconds: 0.2
    connections:
      - role: master
        name: DB-Writer
        host: localhost
        port: 5432
        blacklist_duration: 0
      - role: slave
        name: DB-Reader
        host: localhost
        port: 5433