src: remove redundant symbol init in node_file.cc

Don't lazy-init the oncomplete symbol string in the After() function,
it has already been created at module init time.
http: speed up callbacks, use array indices

Use array indices rather than named properties to store callbacks on
the HTTPParser object.  Speeds up the http benchmarks by a few percent.
tls: handle errors on socket before releasing it

Fix sudden uncatchable ECONNRESETs, when using https server.
zlib: replace C cast with static_cast

Seems to have escaped the Eye of Sauron^WFedor in commit 8e29ce9.