By default, as RFC 7540 calls for, pushed stream depends on its
associated (parent) stream.  There are some situations that this is
sub-optimal.  For example, if associated stream is HTML, and server is
configured to push css and javascript files which are in critical
rendering path.  Then the default priority scheme is sub-optimal,
since browser typically blocks rendering while waiting for critical
resources.  In this case, it is better to at least give pushed stream
the same priority of associated stream, and interleave these streams.

This change gives pushed stream the same priority of associated stream
if pushed stream has content-type "application/javascript" or
"text/css".  The pushed stream now depends on the stream which
associated stream depends on.  We use the same weight of associated
stream.

This reverts commit e278893b.

SSL_CONF_CTX functions are not working correctly with OpenSSL 1.0.2.
It requires 1.1.0 to make it work.

This is required to avoid creation of temporary ImmutableString
like so:

std::string x;
ImmutableString y = ...;
StringRef ref = !x.empty() ? x : y;

First, temporary ImmutableString is created with x since
ImmutableString has constructor to accept std::string.  After
StringRef gets this, the temporary ImmutableString is destroyed, and
ref has dangling pointer.

This might help throughput, but it interfere stream priority.  The
throughput issue is generally caused by the small buffer size to store
response body, which was 16K.  We increased it to 128K to compensate
this change.

Because of zero initialization, these assignments are unnecessary.

We won't add operator[] to StringRef.  This is because it may be
undefined if pos == size(), and StringRef's base + len does not point
to the valid region.  This solely depends on the given buffer, so we
cannot do anything to fix.  For workaround, if we need this kind of
operator, we may add it under another name, like char_at(size_type).