[refactor] minimize memmove
rename h2o_buf_t to h2o_iovec_t, so that there would be no confusion with h2o_buffer_t or h2o_vector_t
Merge branch 'kazuho/refactor/rename-h2o_buf_t' into kazuho/refactor/buffer-alloc-pattern
introduce reusemalloc, and use it for allocating buffers
detach the first chunk from h2o_mempool_t
use reusealloc from mempool
reduce memory footprint with reusealloc (merge branch 'kazuho/refactor/buffer-alloc-pattern')