http: extendable conn/req ctx and multiplexing

http: extendable conn/req ctx and multiplexing

multiplexing support:
- set request index as connection_index in application session
- do not allocate app session/http req immediately in
  http_ts_accept_callback but wait for request/stream opening

HTTP version specific data in connection and request ctx:
- opaque pointer in http_conn_t
- req_pool manged entirely by specific version engine

Version specific configuration:
- added name to http_engine_vft_t
- added unformat_cfg_callback (optional) to http_engine_vft_t

Type: improvement

Change-Id: Ib43f0489337a222a68b0f81d45cb2e64b2c606c0
Signed-off-by: Matus Fabian <[email protected]>