h2load: Simplify rate mode

This change simplifies rate mode as proposed idea as plan B in GH-382.

In this change, we removed -C option.  Instead, -c option is used to
specify the number of connections to be made, and it is now required
argument if more than 1 clients are required (this is usually the
case).  The number of requests made per connection is calculated
simply by -n / -c.

-n option is handled specially when --timing-script-file is used.  If
-n is used with --timing-script, it specifies the number of requests
-each client will make rather than the total number of requests h2load
-will perform across clients.  This handling applies to rate mode too.

We also clarified the sematics about distribution of rate among the
threads.

src/h2load.h

@@ -82,8 +82,6 @@ struct Config {
   // rate at which connections should be made
   size_t rate;
   ev_tstamp rate_period;
-  // number of connections made
-  size_t nconns;
   // amount of time to wait for activity on a given connection
   ssize_t conn_active_timeout;
   // amount of time to wait after the last request is made on a connection
@@ -200,7 +198,12 @@ struct Worker {
   bool tls_info_report_done;
   bool app_info_report_done;
   size_t nconns_made;
+  // number of clients this worker handles
   size_t nclients;
+  // number of requests each client issues
+  size_t nreqs_per_client;
+  // at most nreqs_rem clients get an extra request
+  size_t nreqs_rem;
   size_t rate;
   ev_timer timeout_watcher;