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
crypto: LazyTransform on properties, not methods

It needs to apply the Transform class when the _readableState,
_writableState, or _transformState properties are accessed,
otherwise things like setEncoding and on('data') don't work
properly.

Also, the methods wrappers are no longer needed, since they're only
problematic because they access the undefined properties.