Add support for remote cluster multi-tenancy

[elprans]

Currently it is assumed that there is always a 1:1 correspondence between an EdgeDB instance and a remote Postgres cluster. This isn't necessary and makes in-place EdgeDB upgrades hard to implement. A solution would be to partition the Postgres cluster between EdgeDB "tenants" by mangling database names using a tenant id, which would be passed as a command-line argument to edgedb-server alongside --postgres-dsn. This approach can also be used to host an arbitrary number of EdgeDB instances in a single Postgres cluster.