[agroal#224] Prefill when doing a switch-to or primary change.

Whenever the primary host is changed, by means of an explicit
`switch-to` command or by a primary failure, the connection flushing
is activated. If possible, the prefill should be also restored on the
new server.
As suggested in
<agroal#225 (comment)>
it would be nice to check if the specified new server is the same as
the old one (failure of the primary) or a different one (`switch-to`),
and in the case they are different the prefill is forced.

src/libpgagroal/pool.c

@@ -802,13 +802,14 @@ void
 pgagroal_flush_server(signed char server)
 {
    struct configuration* config;
+   int newPrimary = (int)server;
 
    pgagroal_start_logging();
    pgagroal_memory_init();
 
    config = (struct configuration*)shmem;
 
-   pgagroal_log_debug("pgagroal_flush_server");
+   pgagroal_log_debug("pgagroal_flush_server %s", config->servers[server].name);
    for (int i = 0; i < config->max_connections; i++)
    {
       if (config->connections[i].server == server)
@@ -849,7 +850,10 @@ pgagroal_flush_server(signed char server)
    {
       if (!fork())
       {
-         pgagroal_prefill(false);
+         if (!pgagroal_get_primary(&newPrimary))
+            pgagroal_prefill(newPrimary != server);
+         else
+           pgagroal_prefill(false);
       }
    }