`get_stored_flows()` should filter for `state=installed`

[Vinicius Arcanjo]

As its is, get_stored_flows() calls aren't filtering for state, which can include both pending and deleted that can lead to issues

On blueprint 32:

assuming that when querying for flows it'll filter for state installed to make sure that flows have been installed on switches.

@gretelliz could you adapt this accordingly? You'll need this guarantee when implementing kytos-ng/mef_eline#188