nghttpx: Add accesslog variables to record request path without query

This commit the following variables to construct request line without
including query component:

* $method
* $path
* $path_without_query
* $protocol_version

Copy build_config.rb to build directory because now
build_config.rb.lock is created along side it.

Define toolchain depending on CC environment variable.

This commit fixes the following warning:

RemovedInSphinx30Warning: sphinx.locale.l_() is deprecated.  Please use `_()` instead.

doc: nghttp2_session_send is also affected by max concurrent streams

Better handling of tls and early data buffer

This reverts commit 14dfeee4.

This fixes that nghttpx stalls when TLS handshake and data follow
after proxy protocol header and no extra read notification is
triggered (e.g., just small 1 HTTP request).

Don't send RST_STREAM to idle stream

Further clarify the function also takes into account maximum concurrent
streams.

Closes #691
Closes #817

doc: clarify flow control behaviour for nghttp2_session_send()

The previous wording implied that if send_callback doesn't return
NGHTTP2_ERR_WOULDBLOCK, this function would always send all
submitted requests / data. This is not the case, since flow control can
cause some data to remain queued until WINDOW_UPDATE is received.