resolved: transaction - increase number of retry attempts

With the exponential backoff, we can perform more requests in the same amount of time,
so bump this a bit.

In case of large RTT this may be necessary in order not to regress, and in case
of large packet-loss it will make us more robust. The latter is particularly
relevant once we start probing for features (and hence may see packet-loss
until we settle on the right feature level).

src/resolve/resolved-dns-transaction.h

@@ -100,7 +100,7 @@ DnsTransactionState dns_transaction_state_from_string(const char *s) _pure_;
 #define LLMNR_JITTER_INTERVAL_USEC (100 * USEC_PER_MSEC)
 
 /* Maximum attempts to send DNS requests, across all DNS servers */
-#define DNS_TRANSACTION_ATTEMPTS_MAX 8
+#define DNS_TRANSACTION_ATTEMPTS_MAX 16
 
 /* Maximum attempts to send LLMNR requests, see RFC 4795 Section 2.7 */
 #define LLMNR_TRANSACTION_ATTEMPTS_MAX 3