Make sure we can handle temporarily offline PS when we first connect (#8094)

Fixes https://github.com/neondatabase/neon/issues/7897

## Problem

`shard->delay_us` was potentially uninitialized when we connect to PS,
as it wasn't set to a non-0 value until we've first connected to the
shard's pageserver.

That caused the exponential backoff to use an initial value (multiplier)
of 0 for the first connection attempt to that pageserver, thus causing a
hot retry loop with connection attempts to the pageserver without
significant delay. That in turn caused attemmpts to reconnect to quickly
fail, rather than showing the expected 'wait until pageserver is
available' behaviour.

## Summary of changes

We initialize shard->delay_us before connection initialization if we
notice it is not initialized yet.

pgxn/neon/libpagestore.c

@@ -381,6 +381,15 @@ pageserver_connect(shardno_t shard_no, int elevel)
 		us_since_last_attempt = (int64) (now - shard->last_reconnect_time);
 		shard->last_reconnect_time = now;
 
+		/*
+		 * Make sure we don't do exponential backoff with a constant multiplier
+		 * of 0 us, as that doesn't really do much for timeouts...
+		 *
+		 * cf. https://github.com/neondatabase/neon/issues/7897
+		 */
+		if (shard->delay_us == 0)
+			shard->delay_us = MIN_RECONNECT_INTERVAL_USEC;
+
 		/*
 		 * If we did other tasks between reconnect attempts, then we won't
 		 * need to wait as long as a full delay.