Remove altsvc
Support custom memory allocator

nghttp2_mem structure is introduced to hold custom memory allocator
functions and user supplied pointer.  nghttp2_mem object can be passed
to nghttp2_session_client_new3(), nghttp2_session_server_new3(),
nghttp2_hd_deflate_new2() and nghttp2_hd_inflate_new2() to replace
standard malloc(), free(), calloc() and realloc().  nghttp2_mem
structure has user supplied pointer mem_user_data which can be used as
per session/object memory pool.