h2load: Reservoir sampling

Refactoring include directories

lib: add nghttp2_rcbuf_is_static()

Add a `nghttp2_rcbuf_is_static()` method to tell whether a rcbuf
is statically allocated.

This can be useful for language bindings that wish to avoid
creating duplicate strings for these buffers; concretely, I am
planning to use this in the Node HTTP/2 module that is being
introduced.

Fix bug that forwarded for is not affected by proxy protocol

Duration watcher and warmup watcher is initialised in Worker constructor. Statistic calculation is removed from duration watcher call_back, it's done in free_client.

MAIN_DURATION is initiliazed in Worker constructor, MAIN_DURATION check is removed from two functions because those functions are needed in warm-up phase as well.

MAIN_DURATION is initiliazed in Worker constructor, MAIN_DURATION check is removed from two functions because those functions are needed in warm-up phase as well.

* Adding timing-sensitive load test option in h2load.

* more checks added for parameters

* A worker thread can control its clients' warmup and main duration.

* Changed warmup to an enum variable.

* removed unnecessary call to ev_timer_stop

* assertion is done before starting main measurement phase

* phase variable is implemented only inside the Worker class

* enum to enum class

* else indentation corrected

* check added for timing-based test when duration CB is called explicitly

* New argument is introduced for timing-based benchmarking.

* styling corrections

* duration watcher initialization is pushed back into warmup timeout

* Warmup and Duration timer is moved to Worker instead of clients. Now both timers and phase belongs to the Workers.

* some client functions are modified to return if it's not main_duration phase. client is not destructed but sessions are terminated

* outputs are adjusted for thread.

* Needed to check if a session exist before terminating

* formatting

* more formatting

* formatting

Fix compile error with mruby 1.3.0

fix: typo

Came up in downstream code review by @lucaslago https://github.com/nodejs/node/pull/14239#discussion_r127539852

README.rst: fix typo