Revert "delay the initialization of ostream filters until the first invocation of h2o_send, so that generators may update the response headers after calling h2o_start_response (as was done in b2ca9ea)"

This reverts commit dda1c8dafff1d006b710e9662b061adc1a08268b.
h2o_start_response now accepts a pointer to h2o_generator_t (instead of allocating it) for greater flexibility
implement support for if-modified-since and if-none-match
Merge branch 'kazuho/expr/conditional-get'
update examples/websocket following the changes in core