Fix some typos in the nghttpx how-to doc

This reverts commit 59c78d58.

Just fix unreachable links and remove ndk...

Fix OCSP related error when building with BoringSSL

BoringSSL has no "openssl/ocsp.h" nor most OCSP related APIs used in
shrpx_tls.cc. This commit add ifdefs to disable related code to allow
building nghttp2 with BoringSSL (again).

It's possible to use !defined(OPENSSL_IS_BORINGSSL), but since BoringSSL
defines OPENSSL_NO_OCSP which is more specific, I chose to go with the
latter one.

Fix bug that timing script stalls with -m1

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.