Scale-out storage: PostgreSQL and MySQL read replicas (ST-5 to ST-15)

A data store with readReplicas becomes a replicated store. Writes,
operator-written SQL and everything outside a read scope go to the
primary. JMAP reads before a request's first write, IMAP LIST, STATUS,
SEARCH, SORT and FETCH, POP3 RETR and TOP, DAV GET, PROPFIND and REPORT,
and blob downloads run in a read scope. Only account data (properties,
indexes, change logs, counters, ACLs, blobs, the search index) is read
from a replica; the registry, in-memory values, the task queue and the
rest stay on the primary.

In a scope, the first read picks a replica round-robin among those up
and under the lag limit, and only if it has every change this node has
written or heard of for the scope's accounts: marks come from write
results, the cluster's state-change broadcasts, a sinceState the client
presents, and, with more than one node, Redis. A write inside the scope
sends the rest of it to the primary. A miss on a replica is looked up on
the primary, and a replica error retries the read there and marks the
replica down.

Each node samples lag every second (WAL positions on PostgreSQL; GTID
sets or Seconds_Behind_Source on MySQL), stops reading from a replica
over 5 s and starts again under 2.5 s, and probes a down replica every
10 s. At startup a replica is left out if it's the primary, isn't
read-only, applies out of commit order, or doesn't show a marker written
to the primary within six tries.

replica_tests (postgres, STORE=PostgreSqlReplicated) runs a primary and a
streaming hot standby in containers: tests 9, 10, 12, 13, 14 and 15 pass.

crates/http/src/request.rs

@@ -151,7 +151,13 @@ impl ParseHttp for Server {
                             path.next().and_then(BlobId::from_base32),
                             path.next(),
                         ) {
-                            return match self.blob_download(&blob_id, &access_token).await? {
+                            // inbuxa: ST-6: a download may be served by a read replica
+                            let blob = store::backend::scaleout::replica::replica_read(
+                                access_token.all_ids().map(|account_id| (account_id, 0)),
+                                self.blob_download(&blob_id, &access_token),
+                            )
+                            .await?;
+                            return match blob {
                                 Some(blob) => Ok(DownloadResponse {
                                     filename: name.to_string(),
                                     content_type: req