Add a process time column to DnsRefreshRequest (#1948)

The value of the column would be set to START_OF_TIME for new entries.
Every time a row is read, the value is updated to the current time. This
allows concurrent reads to not repeatedly read the same entry that has the
earliest request time, because they would only look for rows that have a value
of process time that is before current time - some padding time.

This basically fulfills the same function that LEASE_PADDING gives us
when using a pull queue, whereas a task would be leased for a certain
time, during which time they would not be leased by anyone else.

See: https://cs.opensource.google/nomulus/nomulus/+/master:core/src/main/java/google/registry/dns/ReadDnsQueueAction.java;l=99?q=readdnsqueue&ss=nomulus%2Fnomulus

db/src/main/resources/sql/schema/nomulus.golden.sql

@@ -349,7 +349,8 @@ CREATE TABLE public."DnsRefreshRequest" (
     name text NOT NULL,
     request_time timestamp with time zone NOT NULL,
     tld text NOT NULL,
-    type text NOT NULL
+    type text NOT NULL,
+    process_time timestamp with time zone NOT NULL
 );
 
 
@@ -1627,6 +1628,13 @@ CREATE INDEX idx1rcgkdd777bpvj0r94sltwd5y ON public."Domain" USING btree (domain
 CREATE INDEX idx2exdfbx6oiiwnhr8j6gjpqt2j ON public."BillingCancellation" USING btree (event_time);
 
 
+--
+-- Name: idx3i7i2ktts9d7lcjbs34h0pvwo; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX idx3i7i2ktts9d7lcjbs34h0pvwo ON public."DnsRefreshRequest" USING btree (process_time);
+
+
 --
 -- Name: idx3y3k7m2bkgahm9sixiohgyrga; Type: INDEX; Schema: public; Owner: -
 --