Use Postgres by default

[aaronraimist]

In preparation for #2917, the Debian package and other default ways of installing Synapse should start using Postgres by default

[turt2live]

Related: #2317

[richvdh]

I'm not convinced that this is a thing that needs to happen in preparation for #2917.

One of the problems with it is that it means that, even to try out synapse, you first have to go and set up a postgres database. Something like #2317, where you are given the option of which db to use (and we can provide appropriate warnings about sqlite), might be better. In any case, this just amounts to changing the default option once #2317 is done.

[aaronraimist]

I'll go ahead and close this in favor of #2317 assuming there will be sufficient information provided about the benefits of Postgres at the time the user makes the decision.