shrpx: Explicitly hold server SSL_CTX and client SSL_CTX
shrpx: Refactor spdy downstream header field handling
shrpx: Send RST_STREAM when downstream becomes stale
shrpx: Add --spdy-bridge option

With --spdy-bridge option, it listens SPDY/HTTPS connections from
front end and forwards them to the backend in SPDY. The usage will be
written later. This change fixes the crash when more than 2
outstanding SpdyDownstreamConnection objects are added to SpdySession
and establishing connection to SPDY backend is failed.
shprx: Add --backend-http-proxy-uri option

Specify proxy URI in the form http://[USER:PASS]PROXY:PORT. USER and
PASS are optional and if they exist they must be properly
percent-encoded. This proxy is used when the backend connection is
SPDY. First, make a CONNECT request to the proxy and it connects to
the backend on behalf of shrpx. This forms tunnel. After that, shrpx
performs SSL/TLS handshake with the downstream through the tunnel. The
timeouts when connecting and making CONNECT request can be specified
by --backend-read-timeout and --backend-write-timeout options.
shrpx: Documented --spdy-bridge
shrpx: Check return value of HttpsUpstream::resume_read()

Currently, resume_read() fails if on_read() returns -1 in case that
evbuffer_add failed, which means, most likely, memory allocation
failure. ClientHandler is marked "should be closed", but if
evbuffer_add is failed, write callback will not be invoked and its
marking is not evaluated. It will eventually be deleted when the
client is disconnected or backend failure though.
shrpx: Remove x-forwarded-spdy header field
shrpx: Remove x-forwarded-proto header from SPDY downstream

SPDY frame has :scheme header field, so x-forwarded-proto is not
necessary.
shrpx: Fix client mode does not work
shrpx: Log stream ID when submitting RST_STREAM to downstream