vector-search: reset DNS timeout after changing host

The vector-search client in ScyllaDB limits itself to doing one DNS
lookup per 5 seconds. However, when the configuration changes to point
to a different host, the DNS lookup should happen immediately, and
this patch makes it do that.

Before this patch,
     test/cqlpy/run test_vector_search_with_vector_store_mock.py

Takes a whopping 34 seconds, more than 4 seconds per test!
The problem is that each test creates a new mock vector-store server
and reconfigures Scylla, and when reconfiguring Scylla nothing happens
until the 5-second clock runs out.

After this patch, the same test run is down to 5 seconds.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

vector_search/dns.hh

@@ -41,6 +41,7 @@ public:
     void hosts(std::vector<seastar::sstring> hosts) {
         _addresses.clear();
         _hosts = std::move(hosts);
+        _last_refresh = {};  // reset throttle so the refresh happens immediately
         trigger_refresh();
     }