Implement db-uri-read-replicas

[robx]

I'm still quite unsure about a bunch of this, but felt I needed to get some version of this out the door. It should work in principle but I have less confidence than I'd like mostly because it's quite awkward to test.

The major things outstanding items (in part perhaps those can be left for an iteration):

• There's a lot of complexity that comes with having multiple database addresses
  • Instead of one connection that can be up or down, there are multiple. How do we deal with this in terms of considering things healthy? E.g. if one or more read replicas fail, we could degrade smoothly by automatically falling back to the main server.
  • How do we balance connection pooling between different database; what should happen when we have two read replica and one is very slow
    For a start I went with the simplest approaches I could come up with.
• Testing is hard. I tried a couple of approaches of simulating read-only connections but couldn't make anything work except for a full on replication setup. It would be possible to set this up in integration tests (i.e., I think a variant of withTmpDb that sets up a read replica is doable), but I've shied away from the work so far. It's tempting to spin out the pool implementation to a separately tested package with decent coverage, and leave the postgrest coverage to something minimal.
• The pooling implementation is rather ad hoc, once we settle on a solution it could be nice to break that out into a package

I'll probably think of more once I let this settle for a bit 😅

From the commit message:

• takes a list of URIs to use as read replicas
• queries are routed based on the transaction mode, with read-mode queries going to the read replicas if any are configured
• we configure one pool per database uri, all with the same configuration
• which read replica to use is chosen randomly
• if you want the master to also be used for read-only queries, put it in both db-uri and db-uri-read-replicas

Test coverage is spotty and work-in-progress. It requires a replicating setup which is not automated. With a locally set up read replica, the integration test suite should pass as follows:

PGHOSTREP='/tmp' PGPORTREP=5433 PGHOST='/tmp' PGPORT=5432
postgrest-test-io

You can pass '-k test_readonly' for replication-specific tests.

[steve-chavez]

Oh, what's this used for?

[robx]

That's one of my failed attempts at simulating read-only in tests, missed backing it out. I was calling it as db-pre-request, haven't quite worked out why it doesn't work as intendended. Either it doesn't have the effect I understood it should have, or we (directly in postgrest or perhaps via hasql) reset the transaction mode afterwards.