Upgrade from 1.9.3 to 1.9.4.
Upgrade HTTP/2 implementation patch from v2 to v3.

<ChangeLog>

*) Change: the "proxy_downstream_buffer" and "proxy_upstream_buffer"

directives of the stream module are replaced with the
"proxy_buffer_size" directive.

*) Feature: the "tcp_nodelay" directive in the stream module.

*) Feature: multiple "sub_filter" directives can be used simultaneously.

*) Feature: variables support in the search string of the "sub_filter"

directive.

*) Workaround: configuration testing might fail under Linux OpenVZ.

Thanks to Gena Makhomed.

*) Bugfix: old worker processes might hog CPU after reconfiguration with

a large number of worker_connections.

*) Bugfix: a segmentation fault might occur in a worker process if the

"try_files" and "alias" directives were used inside a location given
by a regular expression; the bug had appeared in 1.7.1.

*) Bugfix: the "try_files" directive inside a nested location given by a

regular expression worked incorrectly if the "alias" directive was
used in the outer location.

*) Bugfix: in hash table initialization error handling.

*) Bugfix: nginx could not be built with Visual Studio 2015.

</ChangeLog>