lib, src: don't make http parser handles weak

Weak handles put strain on the garbage collector and the parser handle
doesn't need to be weak in the first place.  This change should improve
GC times on busy servers a little.
I don't think so.  freeParser() doesn't know where the .parser property lives and there isn't necessarily one.  See for example the call site in lib/_http_server.js where it's a captured variable.