Run Postgres and stellar-core in same container

[SirTyson]

Unfortunately, we cannot run postgres on large clusters like we need to for #287 and #288 due to resource constraints. For postgres, we start a new container for each pod, unlike SQLite. Ops has suggested we run stellar-core and Postgres in the same container to lessen resource requirements. I'm not sure if there is a specific reason to run them separately or not. Otherwise we need to raise cluster limits.

[bboston7]

One temporary fix could be to use postgres only for smaller networks. IIRC we only need postgres for parallel apply, right?