crypto: fix excessive buffer allocation

Allocate buffer only if the next one isn't free.
process: abstract out HandleToStream

Originally contributed by @tjfountaine, but modified to be faster and
more generic.
stream_wrap: introduce StreamWrapCallbacks

StreamWrapCallbacks is a helper class for incepting into uv_stream_t*
management process.
tls_wrap: embed TLS encryption into streamwrap