process: expose NODE_MODULE_VERSION in process.versions
crypto: dh secret should be left-padded

DH_compute_secret() may return key that is smaller than input buffer,
in such cases key should be left-padded because it is a BN (big number).

fix #5239
streams2: coalesce mode and writev
net: switch socket into coalesce mode, _writev
http: write buffers when chunked to embrace writev
test: fix timing sensitive tests